The Word Bank stores words and their definitions across the account rather than per book. Settings lists it under Dictionary; the Companion panel offers it as a tab beside the manuscript. These are one list, not two.
What it is for
A word you want to keep, with the meaning you want to keep for it. Because the list is account-wide, a term collected while writing one book is available in the next.
How it differs from Phrases
Phrases, the tab beside it in the Companion, stores a line rather than a definition, optionally with the plainer version it replaces and a note. A Phrase may also be filed to a particular book, which changes where it sorts. The Word Bank is always account-wide and always a word with its meaning.
How it differs from the Codex
The Codex Glossary category stores invented terms belonging to one book’s world, and those entries participate in tagging and in Name Check. The Word Bank is your own vocabulary, stored across every project, and does not.