The MHRA Style Guide Online
A Handbook for Authors and Editors • Third Edition
From Chapter 6, 'Capitals'
6.7 Small Capitals
Small capitals are specially designed capitals, the height and visual weight of which approximate to those of lower-case letters. They are normally used for roman volume numbers, postal codes (but not abbreviations for US states), professional and academic qualifications, ‘ad’, ‘bc’, ‘ce’, ‘bce’, and ‘ah’. They also provide an alternative to italic and bold type in the typographic treatment of subheadings. For further guidance on roman numerals, see 8.3.
For the presentation of small capitals when preparing copy, see 1.3.3.