Board Thread:General Discussion/@comment-27148615-20151113005149/@comment-6697950-20151113193958

The former is the name used by Dengeki when describing the early Alicization volumes. The latter is officially called the "War of the Underworld chapter".

There's still some content that takes place outside of Underworld in the first sub-arc. Specifically, Prologue II of Volume 9, chapter 2 of volume 10, and two interludes.