The Margo syntax for margin notes defines no keywords. Keywords are instead defined arbitrarily by application developers and other Margo users. To avoid collisions and support interoperability keywords must be reserved through a proposal process. Keywords are intended for use in different kinds of applications, so applications will only use keywords that are relevant to their functionality. That's OK. This is not a programming language.
The reason keywords should be reserved is to avoid collisions between two applications that define the same keyword for different purposes. Another benefit is that two programs that share the same understanding of a keywords can be used interoperably.
Keyword proposals are submitted through pull requests to the proposals directory.
The vocabulary is the list of all accepted proposals. The current vocabulary version is 0.0.1.