Theme lists are fairly small collections of themed words that can be used for creating vocabulary puzzles or as a theme when you fill the grid. Theme lists are stored in a plain text format, and should not usually contain more than a few thousand words at most. Crossword Compiler is supplied with a variety of theme lists that you can use. You can also create your own using the vocabulary puzzle feature, by converting a main Word List, or using AI to generate a list.
The main Word Lists are larger collections of words, can be of any size, and also have a score associated with each word.
Theme Lists are stored in your Crossword Compiler Word Lists/Themes folder. They can be viewed and edited in the Word List Manager. The Word List Manager can also auto-generate a list of words from a description (using an AI model).