What does "shall" mean?

In Maryland there is some debate as to the term shall. Sometimes I like to equate shall with must. Excerpts from this opinion seem to support that position:

Lovero v. Joelma Da Silva., 200 Md.App. 433, 28 A.3d 43 (Md. App., 2011

It is important to note that in each iteration of the rule the clerk is directed not to “file” any paper or pleading requiring service that does not contain the appropriate proof of service. See Rule 1–323 (“The clerk shall not accept for filing....”); Rule 306 a.2. (“The clerk shall not accept or file ....”); and Rule 1(a)(2) (a paper “shall not be received and filed by the clerk ...”). The use of the word “shall” in a rule means that the conduct proscribed is mandatory. See Rule 1–201(a); 5 Owens v. Prince George's Cnty. Dep't of Soc. Servs., 182 Md.App. 31, 43–44, 957 A.2d 191 (adopting the view that the use of the word “shall” with respect to commands found in the Maryland Rules is mandatory), cert. denied, 406 Md. 746, 962 A.2d 372 (2008); Tranen v. Aziz, 59 Md.App. 528, 534, 476 A.2d 1170 (1984) (explaining that the term “shall” in a rule “is presumed mandatory ..., denoting an imperative obligation inconsistent with the exercise of discretion” (citation and quotations omitted)), aff'd, 304 Md. 605, 500 A.2d 636 (1985). Therefore, it is clear that in adopting Rule 1–323, and its predecessors, the Court of Appeals intended that a

        [28 A.3d 50]

pleading or paper requiring service that did not contain the appropriate proof of service was not to become a part of any court [200 Md.App. 446] proceeding by being “filed” in the court file of such proceeding.
Lovero v. Joelma Da Silva., 200 Md.App. 433, 28 A.3d 43 (Md. App., 2011)