Function plan_sign_with 

pub fn plan_sign_with(
    share: Share,
    costs: &Costs,
    mhz: f64,
    shared_sha: bool,
) -> Option<ModulePlan<Profile>>

Best signing module within a share; with shared_sha the module has no SHA rings and sends its three blocks per signature to the shared SHA-512 pipeline (a client port instead of rings, sha = 0 in the profile).

“Best” is the highest rate; builds within 1 signature/s of the best are treated as equal and the one with fewer LUTs wins. Both multipliers are tried when the cost table offers the lean one. None if nothing fits.