[W]hat does Decidable mean?
REVERSE: In logic we state that a question (statement) is DECIDABLE if an algorithm (set of operations) exists within the limits of the system (rules, axioms, theories) that can produce a decision (choice). In other words, if the sufficient information for the decision is present within the system (ie: is decidable).
OBVERSE: Instead, we should determine if there is a means of choosing without the need for additional information supplied from outside the system (ie: not discretionary).
Or in simple terms, if DISCRETION is unnecessary, a proposition is decidable.
This separates reason (or calculation in the wider sense) from computation (algorithm).