Accounts Receivable - Inputs required for Enhancement

Hello Team,

I want to write a user exit for the purpose of writing validation in reference key1 field in customer line item. I will require this validation for:

1. FB70 and FB75 (FI - Invoice posting / debit notes / credit notes)

2. F-28 and F-29 (Downpayment reciepts and receipts against invoices)

3. Posting noted items (F-49)

4. Automatic populating Reference Key1 field during partial clearing, etc.

Please let me know, how to proceed with logic writing in FS. Will I require any BAPI / BADI to be given to ABAPer or I should just give TCode.

Please help with your inputs

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Meena