“PROPER” is a scene tag. As defined in the scene rules, whoever releases the first version of a release – for instance, the Telesync of a film – has won that ‘race’. However, if the release is nuked, or there is some other problem with it, another group can release a PROPER.
“PROPER” is the probbably the most subjective tag within the scene, since a lot of people will generally argue whether the PROPER is better than the original release. This is especially true when the nuke or PROPER reason is trivial, such as the orignal being 4MB under the minimum size, as allowed by the scene. Naturally, the release is still good for most end-users, and a PROPER may not be well distributed. In the same way, a PROPER may have a worse quality source, so may be lacking in areas where the original was not.
A few cheeky groups release PROPERS just out of desperation for not getting their version out first. A reason for the PROPER should always be included in the NFO, and can vary from “Previous release oversized” to “old.x264.version.used” – or any other nuke reason.
If a group wish to release something that does not conform to the scene rules, they can also release an “INTERNAL”, which should not be confused with a PROPER. It is not possible to PROPER an INTERNAL.
Hope this helps