Substitution required for BSEG field updation.

Hi All,

I am having a scenario for updating the BSEG-RECID field depending on the value of the RSEG-GEBER field in MIRO.

When I am trying to create a substituion in GGB1 T.Code:field BSEG-RECID is not displayed there for creating the substitution

How to get that field for applying the substituion in to that screen. Remaining fields in BSEG table are being displayed.

Help me out.

Regards

Srikanth