Because relational databases handle I/O at the column (field) level, and COBOL handles I/O at the record level, some mapping is necessary to associate records with their fields. One function of Acu4GL is to map COBOL records into database fields, and to map the database fields back into records. Acu4GL does this by consulting XFDs generated by the Compiler. The detailed structure of XFDs is discussed in How XFDs are Formed.
Acu4GL builds its own database queries dynamically whenever an input or output request is received. You just compile your program, and Acu4GL does the rest.