A PPC story is any story about events and people occurring in the multiverse of the Protectors of the Plot Continuum. This includes missions, interludes between missions, stories about Infrastructure or Security departments, stories about PPC history, and anything else that occurs in-continuity or that can be considered to have actually happened. Permission is required to write PPC stories.
Things generally not considered to be PPC stories in the sense of requiring permission, even if they involve PPC characters, include random role-plays, MSTs, filks, agent backstories or pieces occurring before they've started working in their chosen department, prompt responses for a Permission request, or anything written for a game on the Board or in the chat. Things like Intelligence reports, essays, and other non-narrative works are also not PPC stories per se, though they can exist in-continuity if people like them enough.