Let us suppose you had never been to a Mass before – in fact you had only ever heard that such a thing existed somewhere – where would you go to try to recreate how to “do” a Mass? The question might be trickier than you think. The information is not all in one place. There is the General Instruction of the Roman Missal, the rubrics, the various decrees, canon law, custom, and various other sources all coming together to inform a person how Mass should be celebrated. (Let alone what type of Mass one might be celebrating.) That is why it is often taught by walking it through and becoming familiarized with a teacher rather than simply studying every last source on how to say Mass.
Perhaps what most people are aware of are the rules called rubrics. The etymology of this word comes from the red ink once used by scribes to denote directions in a text used in the celebration of a sacrament. You can hear the word “ruby” in there can’t you? Today in our liturgical texts we still use the red ink to give directions to the celebrant and other ministers.
The words in red ink are not arbitrarily chosen. They are very concise. If they give a direction without qualifiers they are things that must be done. Any words such as “should” or “may” carry much less weight though should is stronger than may. Order is also important. What is listed first is generally regarded as more desirable than what comes second or third.