A Vesting Certificate is a means of guarding against changes in septic regulations before you build. A plan check approved septic construction permit (SEP) can be vested for 3 years. A Vesting Certificate transfers with the property if it is sold.
A septic design application (SDA) is the review and approval of an OWTS without actual construction details, means, or methods.
Both can be used to confirm the number of bedrooms a site can support.