An IPA Source customer asks:
Is one of them more extensive than the other?
The Ordinary is the invariable part of the Mass such as the Kyrie, Sanctus, and Agnus Dei are part of every Mass. The Requiem is a specific type of Mass for the dead that includes the other “variable” parts of the liturgy such as the introit, tract, and requiescat in pace. Since it is variable, composers may set different parts of The Requiem to music.
Thanks for your question. I hope this helps.