What does CHAIN ....END CHAIN do

Sometimes you want to check several fields as a group. To do this, include the fields in a
FIELD statement, and enclose everything in a CHAIN-ENDCHAIN block.

Example

**** Screen flow logic: ****
CHAIN.

FIELD: SPFLI-CARRID, SPFLI-CONNID.
MODULE CHECK_FLIGHT.
ENDCHAIN.
When an error is found inside a chain, the screen is re-displayed, and all fields found
anywhere in the chain are input-enabled. All non-chain fields remain disabled.