When supportive artists are invited to work on an album they are featured on the record. It is more like a description of what happened rather than a status. The lead artist is the one responsible for the creation of the album or at least developed the inital idea/concept for the album.
However artists can agree on different forms of attribution, like "Artist XY & Artist Z" or with many artists collaborating, a comma-separated list might be used. There is a legal basis for that in most national copyright laws (or equivalents) which specify that the creators determine how the attribution will look.