DBT are storage for fulltext data. Data is stored in simple 512 byte blocks. If data is > 510 byte it will be stored in continuous blocks.
Only reference to the data is an pointer stored in the memo field in the datafile. The pointer is not displayed in dBASE.
If the pointer in a data file record is not empty dBASE will open a file with same name as the data file but with the extention .dbt.

The memo file header has only two values in its 512 bytes:

Rest of the header is garbage.

Next available block will always point the end of the file. Data can only be appended.

The only way to "compressing" a full text file and remove the deleted blocks is by using the COPY command.

dBASE III+

    _______________________  _______
 0 | Number of next        |  ^  ^
 1 | available block       |  |  |
 2 | for appending data    |  | Header
 3 | (binary)              |  |  |
   |-----------------------| _|__v__
 4 | ( Reserved )          |  |
   |                       |  |
   |                       |  |
 7 |                       |  |
   |-----------------------|  |
 8 | ( Reserved )          |  |
   :                       :  |
15 |                       |  |
   |-----------------------|  |
16 | Version no.  (03h)    |  |
   |-----------------------|  |
17 | (i.e. garbage)        |  |
   :                       : First block
   :                       :  |
511|                       |  |
   |=======================| _v_____
512|                       |  ^
   |                       |  |
   |                       | 512 Bytes
   |                       | text blocks
   :                       :  |
   :                       :  |
   |                       |  |
   |-----------------------| _|_____
   | Field terminator (1Ah)|  |  ^
   |-----------------------|  |  |Terminating field
   | Field terminator (1Ah)|  |  |within the block *1
   |-----------------------| _|__v__
   : ( Unused )            :  |
1023                       :  |
   |=======================| _v_____
   |                       |  ^
   |                       |  |
   |                       | 512 Bytes
   |                       | text blocks
   :                       :  |
   :                       :  |
   |                       |  |
   |                       | _v_____
   |=======================|
  1. field terminator Is reported to use only one field terminator (1Ah) - (FoxPro, Fox??)
A memo field can be longer than the 512 byte block. It simply continues through the next block. The field is logically terminated by two End-of-file marks in the field. The reminder of the block is unused.
Every time you re-write a memo field to the memo file in dBASE the field is APPENDED to the memo file (i.e. the file expands for each update). dBASE V for DOS (and perhaps others) may reuse the space from the deleted text, if memo size <= allocated number of block in dbt file. 
There is NO marking of deleted data. Valid data can only be determined from the data file.
The Memo file itself tells nothing about it's use. You need the corresponding DBF file to interpret the content of the memo file. Many of the clones and dBASE 5 have binary elements stored in the memo file marked with file type B or G.

dBASE IV

    _______________________
 0 | Number of next        |  ^
 1 | available block       |  |
 2 | for appending data    | Header
 3 | (binary)              |  |
   |-----------------------|  |
 4 | ( Reserved )          |  |
   | Size of blocks      *1|  |
   |                       |  |
 7 |                       |  |
   |-----------------------|  |
 8 | DBF file name         |  |
   | without extention     |  |
   :                       :  |
15 |                       |  |
   |-----------------------|  |
16 | Reserved   (00h)      |  |
   |-----------------------|  |
17 | ( Reserved )          |  |
18 |                       |  |
19 |                       |  |
   |-----------------------|  |
20 | Block length in bytes |  |
21 |                     *4|  |
   |-----------------------|  |
22 | ( Reserved )          |  |
   |                       |  |
   | (i.e. garbage)        |  |
   :                       :  |
   :                       :  |
511|                       |  |
   |=======================| _v_____
  1|                       |  ^                 Used block
   |                       |  ^           __  |=======================|
   |                       |  |          /   0| ( Reserved )          |
   :                       :  |         /    1|                       |
   :                       :  |        /     2|  FFh FFh 08h 00h      |
   |                       |  |       /      3|                       |
511|                       |  |      /        |-----------------------|
   |=======================| _|_____/        4| Length of memo field  |
  1|                       |  |              5|                       |
   :                       :  |              6|                       |
   :                       :  |              7|                       |
   |                       |  |               |-----------------------|
511|                       | _|_____         8| Memo data           *2|
   |=======================|  |     \         :                       :
   |                       |  |      \       N|                       |
   |                       |  |       \_____  |=======================|
   |                       |  |
   |                       | 512 Bytes
   |                       | text blocks
   :                       :  |
   :                       :  |                 Unused block
   :                       :  |           __  |=======================|
   :                       :  |          /   0| Pointer to next free  |
   :                       :  |         /    1| block                 |
   :                       :  |        /     2|                       |
   |                       |  |       /      3|                       |
511|                       |  |      /        |-----------------------|
   |=======================| _|_____/        4| Pointer to next used  |
  1|                       |  |              5| block                 |
   :                       :  |              6|                       |
   :                       :  |              7|                       |
   |                       |  |               |-----------------------|
511|                       | _|_____         8| ( Reserved )          |
   |=======================|  |     \         :                       :
  1|                       |  |      \       N|                       |
   |                       |  |       \_____  |=======================|
   |                       |  |
   :                       :  |
   |                       |  |
   |-----------------------| _|_____
   | Field terminator (1Ah)|  |  ^
   |-----------------------|  |  |Terminating field
   | Field terminator (1Ah)|  |  |within the block *3
   |-----------------------| _|__v__
   : ( Unused )            :  |
511|                       :  |
   |=======================| _v_____
   |                       |  ^
   |                       |  |
   |                       | 512 Bytes
   |                       | text blocks
   :                       :  |
   :                       :  |
   |                       |  |
   |                       | _v_____
   |=======================|
  1. Size of blocks in memo file (SET BLOCKSIZE). Default is 512 bytes (FoxBase, dBASE IV ??) .
  2. End of text mark is 0Dh 0Ah and line breaks are 8Dh 0Ah
  3. Field terminator Is reported to use only one field terminator (1Ah) - (FoxPro, Fox??).
  4. dBASE III files are marked as lenght = 1.
A memo field can be longer than the 512 byte block. It simply continues through the next block. The field is logically terminated by two End-of-file marks in the field. The reminder of the block is unused.

WARNING ! ! !  DO *NOT* attempt to treat these binary data as simple text ! ! !