BACS is the one scheme that doesn't support the day_of_month / month subscription parameters at all. For every other scheme (SEPA Core, ACH, BECS, PAD, Autogiro, Betalingsservice…), the plugin sends day_of_month (and month, for yearly) directly in the subscription-create call. For BACS, it instead computes a concrete start_date from your day/month preference and sends that — GoCardless then just recurs the subscription forward from that date at the chosen interval. Functionally you get the same outcome, but if you're inspecting the raw API payloads while debugging, don't be surprised that a BACS subscription has a start_date and no day_of_month, while a SEPA one has the reverse.