Standard & user-defined actions modules

Where JIT-actions are defined and where the code to carrying them out?

There is function pool JIT04.
Include LJIT01CON keeps constants of the standard actions:

and the FM JIT04_SET_ACTION_INTERN keeps the code:

User-defined actions for JIT inbound are under Enhancement JIT04_01 (transaction SMOD):

So components of JIT04_01 are FM exits EXIT_SAPLJIT04_001, EXIT_SAPLJIT04_002 and EXIT_SAPLJIT04_003. You can use for own code their includes ZXJIT0U01, ZXJIT0U04 and ZXJIT0U03 accordingly (variable action_iv keeps the current action).

The very similar structure for JIT outbound.
There is function pool JITOUT04 with include LJITOUT01CON.
(JITOUT04_SET_ACTION_INTERN)

User-defined actions for JIT outbound can be implemented into BAdI interface IF_EX_JITO_ACTION (BAdI JITO_ACTION):

The shortcut transaction for it is S_KA5_12001363.