The back bust measurement is a half-measurement based on the full bust measurement. It runs along the back and half way underneath the arm pit.
Terminology - Dresspatternmaking
Pattern grading - Wikipedia
Top 9 Dress Forms
A Complete Course in Dressmaking/Lesson 6/Pattern making - Wikisource, the free online library
Luna Loungewear Sew Along Day One - Love Notions Sewing Patterns
3 Ways to Take Measurements (For Women) - wikiHow
Sewing Measurements & Taking Measurements for Sewing
The perfect dressmaking system (1914) - Wikisource, the free online library
Types of measurement, Patternmaking and Tailoring Wiki