What does "frequency" entail?

The "frequency" does not have any impact from a validation perspective. This means, that even though you have set the frequency to 12, then you can execute payments with only 14 days interval. But this would be misleading for the customer, that they can see in the app, that they are paying monthly, but they in essence are paying every 14 days.

The focal point is, that the "frequency" provides informational value to the customer. The frequency does not impact, how often you can send payments.

What you need to focus on is, that you can send payment requests earliest 32 days before due date and the customer has to be able to see the payment at least 8 days before Due Date. 

You can send the Paym,ent Request 8 days before due date.  : [32 days before Due Date >= Payment Request Date >= 8 days before Due Date].

 The frequency does not impact this. The most frequent subscription payment you can have is bi-weekly.