What do the src and sra form fields for PayPal IPN HTML form do?

Question

Example: paypal recurring payments form

When I have one enabled, it changes "for one month" to "for each month" on the checkout form, so why do I need two of them?

I've looked for the variables here and cannot find it. https://developer.paypal.com/webapps/developer/docs/classic/ipn/integration-guide/IPNandPDTVariables/#id091EB0901HT

Answer 1

You can see descriptions for those (and any other available parameter) in the PayPal Standard Variables Documentation.