The word is optional, even in hosted implementations, for at least one supporting standard.

The identifier is used only as part of a _Pragma expression or #pragma directive.

The word is a type qualifier.

The word is a storage-class specifier.

The identifier exists even in a free-standing implementation.

The identifier might exist even in a free-standing implementation.

[: But surely, everything not guaranteed in a free-standing implementation could potentially exist.]

The identifier is guaranteed to be a macro if defined.

The word is used to specify a type.

The identifier is a function, or a function-like macro.

The word is used to form a preprocessing directive.

The word acts as an operator.

The word or punctuation is native to the language, and does not require any standard header to be included before use.

The word is a keyword. Implies Native.

The identifier is not native to the language, but exists even before any standard headers have been included, under the conditions it is defined to exist.

The identifier is to be optionally defined by the user, and influences subsequent declarations in standard headers.