It depends on the dialog itself.
- For dialogs that block the main thread (e.g. Edit Metadata Dialog), action chains will naturally wait until the dialog finishes and hands back control of the main thread.
- For dialogs that do not block the main thread (e.g. progress bar dialogs, but other dialogs might do the same), but instead act from another thread, action chains will immediately proceed to the next action, even before the dialog finishes. This is almost always undesirable, hence, the option in Calibre Actions to wait for the dialog until it finishes. (see attachment).
Note that this involves guesswork, and is sometimes susceptible to breaking as per the warning in the screenshot.