CSD files describes some CICS mainframe objects, these objects can be used in source code, like for example Cobol and JCL, mainly through CICS statements

File

DEFINE FILE(FILENAME)
DSNAME(DATASETNAME) RLSACCESS(NO)
LSRPOOLNUM(1) READINTEG(UNCOMMITTED)

This is a description of a CICS file named FILENAME and a dataset named DATASETNAME

A link from CICS file FILENAME to dataset DATASETNAME will be created

Transaction

DEFINE TRANSACTION(TRANSACTIONID) GROUP(CARDDEMO) PROGRAM(PROG)

This is a description of a CICS transaction named TRANSACTIONID

A link from CICS transaction TRANSACTIONID to a program identified by PROG will be created, where PROG can be a Cobol, Asm, RPG, …

Program

DEFINE PROGRAM(PROG) TRANSID(TRANSACTIONID)

This is a description of a CICS program named PROG

A link from CICS transaction TRANSACTIONID to a program identified by PROG will be created, where PROG is not the CICS Program object but can be a Cobol, Asm, RPG, …