Or something that many can’t service. Ive seen legacy apps in PROD for which only the binary existed and nobody was exactly sure what it was doing, they had an idea and workarounds for when it breaks.
I think you could make a good definition based on the institutional knowledge of a system, and how much of the system is not included in that knowledge.
Flashback to a utility that only existed as a binary, but was key to some prod processes. Even better - by the time I met it, said utility crashed on a SEGV after it accomplished it's task. So SOP became - ok, xxx crashed, we're good to proceed.