I had the same question and stumbled onto the right solution. You'd think it would be such a common use that it would be a dedicated action in the program & explicitly discussed in the help or forums. But it isn't.
The best way to answer the question is to describe what I did.
I have an audiobook, The Greatest Show on Earth, that consists of 14 audio files, all named like "1 of 14.mp3". The %Album% tag was correctly populated as "The Greatest Show on Earth" and the %Track% tags were correctly numbered. But the title tags were all of format "%track% of 14". I wanted each title to be of the format:
But there was no obvious way to automatically rename each Title using the value of other tags. MP3Tag's ACTION menu must be used:
Select all the tracks in the track listing pane.
Select Actions->Actions menu item. This opens the Action Groups dialog box.
Click on the "Star" icon to create a new action group. Enter a name for the Action Group; I used "Rename Title". This opens the Actions dialog box.
Click on the "Star" icon to create a new action.
Select "Format value" as the action type. Click OK.
This opens a dialog in which to select a target tag (%title%) and specify source tags to define. Oddly, in MP3Tag 2.48, TITLE is not one of the options in the drop list. TITLESORT is, but this IS NOT the tag you want. Fortunately, you can enter TITLE as the target tag manually (it will try to autocomplete TITLE as TITLESORT; hit the delete key to override).
In the next field, "Format String", enter:
Click OK. This brings you back to the Actions dialog.
Click OK. This brings you back to the Action Groups dialog.
Check the action group that you just created; in my case, it was Rename Title.
Viola. The %title% tags have been automatically renamed.
Hope this helps future users with the same question.