In addition to manual spell checking for individual topics, selected text and your entire project you can also use live spell checking (misspelled words are highlighted as you type). You can create your own user dictionaries with lists of special terms for your own projects, auto-correct entries and terms that you want to identify as incorrect even though they are in the main dictionary. User dictionaries can be global for all projects or local for just one project and can be stored anywhere you like.
See The Spell Checker for full details on configuring the spell checker.
Spell checking is supported almost everywhere in Help+Manual where you can enter text. Just right-click to display the context menu or click on the upper half of the Spelling tool in the Project tab to access.