When you are logging on to CICS, you are in the 0 level in CICS. That is the highest level. When you run a transaction, your transaction works under CICS, that is your transaction is in level 1. When the transaction issues a LINK, the linked program works under level 1, i.e. under the main program. So the linked program is in level 2. If the main program issues a XCTL, the XCTLed program works in the same level as the main program, i.e. level 1 itself. Similarly when the main program issues a RETURN , the RETURNed program also works in the same logical level as that of the main program.