A reason why a package can't be used as input to the solver. e.g. it is for a different platform, or conflicts with a user-provided constraint.

candidates t name is the list of available versions of name, in order of decreasing preference. If the user or environment provides additional constraints that mean a version should be rejected, include that here too. Rejects are only used for generating diagnostics reports. Candidates whose "availablity" field isn't satisfied must be rejected here.

user_restrictions t pkg is the user's constraint on pkg, if any. This is just used for diagnostics; you still have to filter them out yourself in candidates.

filter_deps t pkg f is used to pre-process depends and conflicts. pkg is the package which has the dependency f. For example, you can use this to filter out dependencies that are only needed on Windows if the platform is Linux.