Divisions always need titles, you accomplish this with a <title> tag first thing. Almost everything that you can use in a paragraph can be used in a title, but a few constructions are banned, such as a displayed mathematical equation (for good reason). Try to avoid using footnotes in titles, even if we have tried to make them possible.
Lots of other structures admit titles. Experiment, or look at specific descriptions of the structure you are interested in. Titles are very integral to PreTeXt, much like cross-references. Titles migrate to the Table of Contents, get used in page headers for print output, can be used in lists (such as a List of Figures), and can be used as the text of a cross-reference, instead of a number. You might not be inclined to give a <remark> a title, but it would be good practice to do so. Your electronic outputs will be much more useful to your readers if you routinely title every structure that allows.