I just wanted to know for a collaborative song, what are the factors affecting which artists to be credited as the lead artists and which artists to be credited as the featured artist.
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.