Short answer
Subclass 300 is a temporary visa for a person outside Australia who intends to come to Australia, marry their sponsor, and then apply for a Partner visa in Australia.
The subclass 300 visa is for prospective spouses. It is designed for a couple who intend to marry after the visa is granted.
It is not the same as a Partner visa already based on an existing spouse or de facto relationship. The applicant needs to satisfy the subclass 300 criteria, including the intention to marry and sponsorship requirements.