PGrid is just the name used in the presets which reflects the new low level phosphor implementation.
While it provides the same functionalities as before, it has the ability to scale the horizontal mask (well, even this functionality was already present), but this time when the mask is scaled so that there is more space between phoshors, they are able to grow in width and eventually blend each other when their light overlaps.
Also, when not using a preconfigured mask preset, this time the phosphors can be freely moved horizontally by sub/phosphor steps, so that you can achieve new masks not possible till now, like xxrgbxx (actually used in handheld preset do have a visible grid around them.
Or, you can move r g and b to the very same position to completely overlap them, clear the visible mask, and have a solid color instead
This is for the horizontal mask ofc, but part of the same implementation is used for level 1 vertical mask, where you emulate scanlines, and level 2 vertical mask, where you emulate aperture grille or slotmask.
Being able to switch from core to screen coordinates in each step, you can achieve different results, the implementation is simple by design, but may be difficult to master, that’s why i suggest you to take a read to the docs-ng.md file, where i tried to explain all the things in detail.