Function evaluate 

pub fn evaluate(
    dir: &Path,
    signers: &Path,
    principal: Option<&str>,
    require_platform: Option<&str>,
    anywhere: &[String],
    include_local: bool,
) -> Verdict

The whole decision, trail included.

principal of None means “whoever the signature says, if that key is in the file”; Some narrows it to one identity. require_platform of None means the caller has stated this suite’s result does not depend on where it ran. anywhere names gates the caller accepts from any platform — its committed statement that THOSE checks cannot depend on where they ran — and only ever admits gates from a block whose signature verified.