A ceremonial commandment or ordinance.

A direction in a liturgical book as to how a church service should be conducted, traditionally written or printed in red ink. Also figurative.

A rubric.

Christian Church. A rule prescribing, or book containing, the order of divine service, esp. that of the Mass; the established order or form for…

Ecclesiastical. A caution or direction for the proper administration of the sacraments; esp. in cautels of the Mass.