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4.2.3.2.2 Assignment Name for Data-Names and Environment Variables
If the environment variable or data-name is specified for the assignment name, the data-name value or the environment variable value is processed as follows: 



    ___ Environment variable and data name value format _______________________ 
   |                                                                           |
   | >>__ _________________ __system file name_______________________________> |
   |     |_file system ID-_|                                                   |
   |                                                                           |
   | >__ _____________________________________________________________ _____>< |
   |    |                         <______________________________     |        |
   |    |_(__alt-inx-file-name-1___ ____________________________ |__)_|        |
   |                               |_,__ _____________________ _|              |
   |                                    |_alt-inx-file-name-2_|                |
   |                                                                           |
   |___________________________________________________________________________|
file-system
ID If the file-system ID is specified explicitly using the environment variable value or the data-name value, that specification for the file system overrides any file system specification made by the ASSIGNment name.
The environment variable value for a file is obtained when the program containing the file is first invoked (or called) in its initial state. This value is kept for the file for subsequent calls to the program in the last used state.
The value of the file ID specified with a data-name is obtained when the file is OPENed. On each subsequent OPEN for the file, the value is reobtained.
File declarations for an external file must have the same file-system identifier. If they are not, the error is caught during run time, and the application is terminated with an error message.
system file
name If there is a hyphen in the environment variable or the data name value, the first three characters to the left of the left-most hyphen are treated as the file-system identifier. The character string to right of the left most hyphen is then used as the system file name (possibly including drive and path names).
If there is no hyphen or the character string to the left of the left-most hyphen is less than three characters long, the entire character string is used as the system file name (possibly including drive and path names).
For information on specifying alternate indexes, see topic 4.2.3.2.1.

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