Obligatory; required or commanded by authority.
Of, being or relating to a mandate.
A sign or line that require the path of the disc to be above, below or to one side of it.
Required as a prior condition of something else; necessary or indispensable.
Something that is required as necessary or indispensable, or as a prior condition of something else.
In education, a course or topic that must be completed before another course or topic can be started. May be colloquially referred to as a prereq.