Using the Pick files feature in the Pro interface you get a good overview of which files have been assigned to an action or album and which remain to be assigned.
To pick files to assign to actions or albums, click on the Pick files button below the list of actions in the right side panel. The Albums picker is located below the list of albums.
Now, when you pick files (keyboard modifier keys can also be used for multi-file selections) they will be marked with a green checkmark after which you can click on the + sign on an action or album to add them to it.
By default the files you have picked will disappear from view. This is very practical if you started out by isolating a selection on a separate tab, since it allows you to easily see which files remain to be assigned. However, it's perfectly possible to use the Pick files mode in a regular archive also.
If you would rather want the files that have been assigned to an action or album to remain in the grid, simply tick the Show picked files box above the Done button in the side panel.
When you've assigned all files to an output or album, click on Done to complete the process.
Now, to process the files, click on the green Play icon next to the queued-up assets on the action. An action may require your input to be complete. If that is the case, follow the on-screen instructions to complete the process.
Tip: You can also just make selections of files and drag them to an action or an album. Using the picker, though, you get a better view of which files have been assigned and which remain.