Click the Close AceText Collection button on the main toolbar, press Ctrl+W on the keyboard, or select Collection|Close Collection in the menu to close the active collection. If the collection was previously saved and has unsaved changes, AceText will silently and automatically save those changes. You can turn off the automatic save in the Files Preferences. In that case, AceText will ask if you want the changes to be saved or not.
If the collection is untitled, AceText will ask you if you want to save it. If you do, AceText will ask you for the location and file name.