Only Promo Credits are used for Playlist Promotion. You earn Playlist Promotion whenever you upgrade to a new rank.
When you upgrade to a new rank, you’ll unlock Playlist Promotion campaigns (never expires) that you can claim at any time for any Spotify release by going to the ‘Playlist Promotion’ section of your dashboard.
Promo Credits are used for Playlist Promotion. Normal credits are used for everything else. You can see how many credits of each type you have on the top right of your dashboard:
Learn more about the difference between the credits here.