Field 'Goods Receipt / Performance of Service' in 'Related Documents' Tab of PO Item in SRM

Hi Gurus,

I have a requirement where I need to make 'Goods Receipt / Performance of Service' in 'Related Documents' Tab as non-editable based on certain fields on the PO Item.

On searching SDN, I found that this can be done via SPRO in the below path SPRO->SRM Server->Cross application basic settings->Extension and field Control -> Configure field control -> configure control for fields at item level.

But here in this I am finding difficulty on using the Dyn. Customer Class and method.

Please help us on how to find the relevant standard CLass and Method.

I am new to SRM Programming.

Appreciate your inputs on this.