The specifications repo has a top level file called "driver read preference":

https://github.com/mongodb/specifications/blob/master/source/driver-read-preferences.rst

This is a rather extensive document, but there is also this section on read preference in server selection specification:

https://github.com/mongodb/specifications/blob/master/source/server-selection/server-selection.rst#read-preference

The two documents seem to cover the same functionality. The top level spec does not use rfc2119 language making it difficult to figure out what the actual requirements it imposes on drivers are. Sections like https://github.com/mongodb/specifications/blob/master/source/driver-read-preferences.rst#errors make me think the top level spec is the older of the two and potentially outdated.

It seems to me that the two specifications should be combined into a single document (or a section under server selection, if this makes more sense).