Most people hear of canon lawyers in today’s world only when they know of Catholics in marriage nullity cases. But as an example of the importance of canon law and canonists, allow me to use the person quoted above as an example, if she or he does not mind.
Aramis is a Byzantine Catholic. As a member of the Catholic communion, Aramis is permitted to receive Holy Communion in any of the 22 or 23 Catholic churches (in full communion with the Holy Father and with each other). But not many Roman Catholics know about the Byzantines, and quite possibly some of the Latin priests. Suppose Aramis presents her- or himself to Holy Communion at a Latin parish but is denied when someone finds out beforehand that Aramis is Byzantine. The first thing to do is to discuss the matter with the pastor or parish administrator after Mass. If Aramis is not satisfied with the response, she or he could take the case to the bishop or ordinary of the diocese or ordinariate. If still Aramis is not satisfied with the ordinary’s response, she or he could consult a canon lawyer and lodge a complaint with the Holy See (there is at least one group of canon lawyers that I know, the St. Joseph Foundation, which deals with non-matrimonial issues).
Don’t mind at all.
I’ve never had to go further than pointing out can. 844. But the fact is, said priest did call his diocese’s canon lawyer to confirm. (The irony - my pastor at that time WAS said canon lawyer… he was biritual.)
To bring up a few other issues - Canon law is very convoluted - specific jurisdictions overlap and abut.
Take, for example, a hypothetical Dominican friar assigned as a pastor and superior of a house. When functioning as a secular pastor, he’s acting under the CIC, the particular law of the ecclesiastical province, and the particular law of the diocese, and any special law applicable to the parish. As a superior of the attached house of Dominicans, he functions under the CIC and under the Dominican Order’s particular law, the provincial particular law, the Dominican Order’s charter, and under the general canon law relating to papal right religious orders. And, when both practical and not sinful, he has also to obey local civil and criminal law while doing so. As a friar, he’s not subject to the particular laws of the diocese or province, but is subject to the CIC, the dominican charter, etc. But, when acting as a pastor, he has to obey it in as much as what rights the faithful have have to be provided for.
And, in dealings with people of different churches (properly, Churches Sui Iuris) within the Catholic Communion, things can get even more convoluted.
Almost all Western religious are papal right; most Eastern are patriarchal or diocesan right, and so may be subject to diocesan law. Western law allows marriages to be witnessed by a deacon; eastern does not, and requires a priest. Roman law allows for a screen for confession, and requires a confessional be available; Eastern does neither - in fact, confessions are often done in the front of the nave. Roman Law requires a divine worship service - The mass or a Liturgy of the Word using the same readings as would a mass - for sunday and holy day obligation; eastern allows communal vespers or matins to fulfil the obligation if it is the tradition of the Church Sui Iuris.