The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 8, 'Dates, Numbers, Currency, and Weights and Measures'
8.3 Roman Numerals
The use of roman numerals should be confined to a few specific purposes:
(a) large capitals for the ordinals of monarchs, popes, etc. (Edward VII), and for major subdivisions within a text;
(b) small capitals for volume numbers of books (journals and series take arabic numerals), also for the acts of plays, for ‘books’ or other major subdivisions of long poems, novels, etc. (see 11.2.7), and for certain documents.
(c) small capitals for centuries in some languages other than English (‘xvie siècle’, ‘siglo xvii’); however, in Cyrillic script large capitals are used;
(d) lower case for the preliminary pages of a book or journal (even if the original uses capitals), where these are numbered separately, and for minor subdivisions within a text; inclusive numbers are written out in full, e.g. ‘xxiv–xxviii’ not ‘xxiv–viii’.