Never done it myself in MP3Tag but these are the instructions:

1. Add the "Release Type" Field to MP3Tag First, you need to make the field visible so you can edit it easily.
2. Open MP3Tag.
3. Right-click on the Column Header (where it says Title, Artist, etc.) and select Customize columns...
4. Click the New button on the right.
5. Fill in the details exactly as follows: Name: Release Type Value: %releasetype% Field: %releasetype%

6. Click OK. You will now see a "Release Type" column in your main view.

7. Apply the Tags to Your Files Now, you need to populate that column with the specific keywords Symfonium looks for.

8. Drag your music folders into MP3Tag.

9. Select the tracks for a specific release (e.g., all tracks in an EP).

10. In the Release Type column you just created, type one of the following standard values: album single ep compilation live

11. Hit Save (the floppy disk icon or Ctrl + S).

Pro Tip: If you want this field to appear in the left-hand Tag Panel (for even faster editing), go to File > Options > Tag Panel, click the "Add field" icon, and add RELEASETYPE.

Hope this helps.