two arrangements in PVG popular music?

A question regarding printed popular music in Piano/Vocal/Guitar format, for example Hal Leonard PVG books like 'Best Songs Ever', and songs like Blue Skies or Bewitched, etc.

It is very common that most of these PVG songs have what I will call (for lack of knowing a better term) dual arrangements printed in some measures. In the same one song, the location of the two sets of different notes switches dynamically, being often in only one or the other piano staff, but occasionally both, or often neither at the moment. This duality is NOT a constant over the whole duration, so it is not what I would imagine to be two parts for two instruments (but I may be wrong, so I am asking?). There are simply two sets of notes in some measures, with both sets counting 4 beats if 4/4 time. One set is drawn stems up and one drawn stems down, but one set is above the other in the same staff measure - for example, there may be a quarter rest ABOVE a half note in the one five line staff measure.

I dont know how to describe it better, but if you have seen it, it seems obvious that you should know exactly what I mean because it is very common - I'd say most Hal Leonard PVG popular music songs, but not all.

A major characteristic appears to be that one set of notes is simpler to play, maybe only one whole note where the other set has a more complex set of notes and chords and rests.

So my wild first guess is that this is simply two arrangements for the price of one, and one might choose to play it either way. However since other simpler versions are available, that doesnt sound logical either. I dont think it is a factor, but comparing the PVG version to an Easy Piano version sees a different third version or arrangement of the same song.

So is there more to it? What is the purpose of the dual sets of notes? How is it used, and does this feature have a proper name?

I have that open in front of me.

<snip> <snip> So is there more to it? What is the purpose of the dual sets of

The 'simple' set of notes is the monophonic vocal line, and the other is instrumental accompaniment. You'll notice that the stave omits the vocal line when there is an instrumental introduction, bridge or conclusion.

I haven't got the Hal Leonard best songs ever. But the non vocal set is usually the solo piano version. It includes the melody, so it's not just an accompaninment. The guitar chords is the accompaniment. If the pianist playing for a singer, he'll play chords according to the 'guitar' part.