In the USFM documentation (
https://ubsicap.github.io/usfm/titles_headings/index.html#ms), \ms# (\ms1 \ms2 etc.) is rather used for sections within a book: it is used when the translator wishes to put titles above sections at a higher level than the standard sections, which have heading \s (or \s1). For example, one could place a \ms above the Sermon on the Mount (Mat. 5-7), while each section within this discourse gets a heading \s.
To my knowledge, there is no dedicated marker for titles above the book level. (I hope someone will correct me if this has changed.)
If \ms is not used otherwise in the translation, a team could decide to use \ms for this. Or if \ms1 is in use within books, the team could use \ms2 for higher-level titles. \ms2 could be given a custom style in the custom.sty file. And when it comes to typesetting, the team can work out an appropriate formatting for this style.
