What is a test pressing

Musik-Sammler.de - Forum

In principle, white labels are all test pressings that are then distributed to radio stations. usually with an enclosed advertising paper, in about half of the cases a band bio is included.
The presentation of the test pressings is usually the white inlay. Sometimes it also comes with a white inlay and white cover.
In the rarest of cases, the original cover is also included. But that is usually not even finished.

Promos usually have a promo stamp, but often we punched a hole in the cover or cut off a corner to reduce the value (radio stations shouldn't make money with the disc). (CC - CutCorner)

As far as the value is concerned, this is twofold and roughly corresponds to the one already mentioned.
Test pressings are much rarer than promos, but often less well equipped and really only for 'rarity hunters', fans of combos and / or completeers.

The promos are different. If it is a CC, the value drops enormously because the disc has nothing special to show (no stamp, etc.). LP with a promo stamp is also only marginally more valuable than a regular one.
But effective promos, i.e. discs that are only available in promo form, are already more sought after. Some US labels (e.g. Combat, Atlantic, etc.) have often released pure promo EP's that contain, for example, a pre-track. At that time, for example, the Savatage live CD From the dungeons to the Streets was very much in demand, which was only released as a promo and since there was no live disc up until the time of Sava, you can imagine how wanted the part was. Or the Manowar Wheels of Fire Pic-Disc, which demanded enormous sums in the 90s. To collapse again in the 2-digit range towards the end of the 90s.

A few years ago I was able to scour part of the metal collection of the CH radio DRS3 or their metal show. There weren't too many promos (especially what interested me). They were mostly regular albums, but they were all in very good condition.