Triggering of enhancement codes upon changes on certain field only

Hi,

Considers the below statements in SAPMV45A, Screen 4900 (PAI module).

    CHAIN.

       FIELD: vbap-matnr,

              vbapd-matnr_g,

              rv45a-mabnr,

* IS2ERP Media enhancement

              vbap-idcodetype,                           "ISBN-Nr.

              vbap-ismidentcode,                         "ISBN-Nr.

              vbap-kdmat,

              vbap-posex

               MODULE vbap-posex_pruefen ON CHAIN-REQUEST.

     ENDCHAIN.

I implemented my code enhancement in module 'vbap-posex_pruefen'.

May I know is there a way to trigger the enhancement codes only when VBAP-KDMAT is changed (while changes on other fields such as vbap-matnr, vbap-posex etc will not trigger it)?

Kindly please let me know if you know how to achieve this.

Thank much, Nick