There are a number of different places in Opus that you can use the evaluator. Most are optional - the evaluator can be used to augment or extend a simpler system - but some functionality requires its use.
Evaluator Columns - define file information columns that use the evaluator to generate their contents.
Filters and Find - filtered file operations and the advanced find tool both let you write evaluation expressions which can be used to select files for operations or return them as part of the search. You can also enter evaluation code directly into the search field when it's set to use Opus Find.
Functions - many command modifiers can use the evaluator, labels and icons can be changed dynamically, and command lines can contain inserted text (or even be generated completely) using the evaluator.
Rename - insert text from the evaluator into the new filename.
Rename Control Keys - define Ctrl key sequences for inline rename that use the evaluator to select, replace or delete text, or to move the cursor.
Status Bar - insert evaluator-generated text into the status bar.
Column Context Menus - the evaluator has special support for column header context menus, so that you can make dynamic menus that change depending on which column was clicked.
Tab Context Menus - the evaluator has special support for folder tab context menus, so that you can make dynamic menus that change depending on which tab is clicked or how many tabs are open.