I consulted my FM 5-15, but there's no Section VII in this version ! Mind you, my copy of FM 5-15 is dated 14 February 1944 - entitled "Field Fortifications" . I assume yours is an early print, dated 1940, or maybe parts of C 1 (1941), C 2 (1941), or was this mentioned in Training Circulars No. 52 (1942) or No. 96 (1943) ?
Contents of Manuals (may) vary over the years, I therefore always find it interesting to consult different issues, so as to learn more about their evolution ...
Other forum members may have Manuals with other dates, also worthwhile to check .