One file a plugin owns or contributes to (SDD 7.3).

mode and key are exclusive. :contributes names one block inside a file the plugin shares with others, and :manual names a hunk that cannot be appended safely and is applied as conflict markers instead. Both are positional inside a file someone else owns, so both REQUIRE a key. :sole_owner and :seed take the whole file and must carry none. A nil key is omitted from the encoded entry rather than written as key: nil, so the bytes match SDD 8.2 exactly.

:manual extends SDD 7.3's three modes. It is not a phantom class like :overwritable: apply acts on it by writing markers and mix sv_ex.check acts on it by failing while one remains.

There is no :overwritable mode. It is SDD 4.1 defect 5/6 — a mode nothing downstream can act on — and it appears here only as a value the last clause of validate!/1 rejects, so it can never become a phantom class.