Section 2.4, "The CredentialRequestOptions", defines the semantics of various options that can be provided with the credentials.get() call. The two properties that are missing are "fields" and "params".

Both should be included in the explanations, with "fields" marked as an extension.
While "fields" is pretty straight forward, the semantics of the "params" property are not as clear cut. The way I understand FedCM, the "params" property is intentionally only loosely defined to allow for profiles/extensions/protocol variations to define and use it. However, the discussion around merging the "nonce" parameter into "params" (see for example this PR) shows that this is not a universal view. As such, some discussion might need to happen before it can be defined.

I could provide a PR, but as I am not a member of the working group and this is a meaningful change to a normative part of the specification, it is probably better if somebody else opens a PR for process reasons.