Short answer
Subclass 309 is the provisional stage of the offshore Partner visa pathway.
The subclass 309 visa is for a person seeking a provisional Partner visa based on their relationship with an Australian citizen, Australian permanent resident or eligible New Zealand citizen.
The relationship may be assessed as a spouse relationship, de facto relationship, or in limited circumstances an intended spouse situation. The related subclass 100 permanent Partner visa is usually linked to the same offshore partner pathway.