obligatory
absolute
authoritative
binding
canonical
choiceless
commanded
commanding
compelling
compulsatory
compulsory
conclusive
de rigueur
decisive
decretory
demanded
dictated
didactic
directory
entailed
essential
exigent
final
forced
formulary
hard and fast
hard-and-fast
imperative
imperious
importunate
imposed
indispensable
inevitable
instructive
involuntary
irrevocable
mandated
must
necessary
necessitous
needed
needful
official
peremptory
preceptive
prescribed
mandatary
receiver
recipient
additional
annual
definitive
federal
minimum
operational
periodic
preliminary
regulatory
statutory
strict
stringent
optional