Stage (which used to be Play years ago) is designed to work out of the box with little or not modification to the built-in preset collection for any instruments you own. So, in practice, you can change a few general settings (volume, dynamics, a few details about the action, etc.) but not much else. So Stage is great if you simply want to open the VST and play without working with settings or custom FXPs.
Stage doesn't really load any FXPs beyond what's distributed with it. Standard allows for many parameters to be changed, but only a few on a note-by-note basis. Pro allows all parameters to be controlled on a note-by-note basis. FXPs made in Pro will open and be honored in Standard but the ability to edit them further isn't there. It might also be that very specific features like Quad-channel Convolution Reverbs are only available in Standard or above (it's been long enough that I can't remember). I do know that microphone placement and multi-channel features are limited to Standard/Pro. Also, the Layering engine isn't in Stage.
If you need a simple set-it-and-forget-it bare-bones VST instrument solution for live performance or practice, Stage is very sufficient (I was fine for about 3-4 years with just Play >> Stage). If you want to build your own sounds, customize granular features, change microphone settings, or layer different instruments, Standard or Pro would be the way to go.
Spotify:
https://open.spotify.com/artist/2xHiPcCsm29R12HX4eXd4JPianoteq Studio & Organteq
Casio GP300 & Custom organ console