SET       --- NEXT DATABASE   =    DB1
    OPEN      TAR CUSTOMER        SHARE? Y FAIL 0
    *
    BEG READ  TAR CUSTOMER        Hold? N
    *
    *         The DB1 Customer file is currently accessible
    *         through the CUSTOMER file name - we just read 
    *         a record from the DB1 Customer file.  Now we 
    *         Want to write this record to the DB2 Customer
    *         file - have to open it first
    *
    SET       --- NEXT DATABASE   =    DB2
    OPEN      TAR CUSTOMER        SHARE? Y FAIL 0
    WRITE     TAR CUSTOMER                 FAIL 0
    F CANCEL unexpected error (duplicate key?) WRITEing CUSTOMER 
    *
    *         Now we have to switch databases again before we
    *         do the END READ
    *
    OPEN      TAR CUSTOMER        SHARE? Y FAIL 0
    END READ  TAR CUSTOMER 
 

When APPX needs to open a file, it does so in two phases... a logical open and a physical open. The logical open takes place when APPX opens the file automatically OR when you open the file with an OPEN statement - this phase sets up internal memory structures to keep track of the file - the structure files are also verified during the logical open (this is expensive).

The physical open only takes place when you actually do the first I/O to the file (a read, write, etc.).

A file name (use "CUSTOMER" as an example) actually acts as a file multiplexor - you can get to multiple CUSTOMER files using the same file name. You do this by setting --- NEXT DATABASE and executing an OPEN statement to open a CUSTOMER file in a different database.