Short answer
Subclass 115 is a permanent family visa for a person who is the remaining relative of an eligible Australian relative.
The subclass 115 Remaining Relative visa is for a person seeking permanent residence because they are the remaining relative of an Australian citizen, Australian permanent resident or eligible New Zealand citizen.
The applicant must meet the remaining relative rules and must keep meeting them when the visa is decided. Changes in family composition or the Australian relative's circumstances can affect eligibility.