You can find the list of Withholding Tax ATC's Here:
If you're entering a Received 2307, then we recommend you choose the same tax code that was chosen by your client.
If you're entering an Issued 2307 (you are the withholding agent), then choose the tax code that is most appropriate. Note that most of these tax codes have a "threshold" (less than P3M / less than P720K / etc). It would be prudent to ask your payee for a notarized affidavit stating that they are earning less than the stated threshold.
The BIR has kindly provided templates for that purpose which you can find here:
PAYEE’S SWORN DECLARATION OF GROSS RECEIPTS/SALES
(For Self-Employed and/or Engaged in the Practice of Profession with Several Income Payors)