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 is to map COBOL records into database fields, and to map the database fields back into records. does this by consulting XFDs generated by the Compiler. The detailed structure of XFDs is discussed in How XFDs are Formed.
builds its own database queries dynamically whenever an input or output request is received. You just compile your program, and does the rest.