1) the pixel grid is the bitmap resolution of any given file open in PhotoLine. If the image is 320x200, then that is the pixel grid. If it is a 300ppi A4 document, the pixel grid will be 2480x3508 pixels.
2) It works with text layers and vector layers. Image layers are already bitmap/pixel based, so there is no visible effect IF and WHEN those image layers are displayed at 100% their native pixel dimensions. But if you scale an image up or down, it will have an effect depending on the settings. The rendering of transformed image layers will be affected by these settings.
3) This is an interplay between anti-aliasing on/off and pixel snapping on/off, as well as activating pixel view mode.
Before I continue this explanation, it is important to understand that PhotoLine's internal transformation system is decimal based. Even with pixel snapping turned on for one or more layers, or the entire document, the positioning will never snap to exact pixels. That is not how PhotoLine works (the same holds true for a number of other image editors out there. (Although it would be nice if PhotoLine actually would have this behaviour AS AN OPTION in my opinion - an option, because the current behaviour does have advantages).
What PhotoLine does in pixel snapping mode is rounding the transformation values to non-decimal values during the rendering. It will not snap the actual transformation /positioning to the pixel grid.
For example: (PS you MUST enter PIXEL MODE (view-->pixel mode) for this to work)
Draw a vector rectangle, yellow with black 1px outline. Turn anti-aliasing on for this layer, and pixel snapping off. Move it. Notice the smooth transition between decimal values and its effect on the overall anti-aliasing. This can be a boon, since it allows us to control how the rectangle's anti-aliasing is affecting the overall look of the edges via decimal positioning.
Now turn on pixel snapping. Keep anti-aliasing activated. Notice how the values are rounded to the nearest pixel. Even though anti-aliasing is turned on, the edges snap to exact pixels, and the rectangle's edges are rendered without anti-aliasing. (A rounded rectangle would still have anti-aliased corners, however). Rotate the rectangle and the anti-aliasing kicks in (as it should).
Now turn anti-aliasing off as well. Notice that no anti-aliasing is used at all, even when rotating the rectangle.
That's how it works in PhotoLine: it's an interplay of pixel view mode, anti-aliasing setting, and pixel snapping. Vector edges that run along the x or y axis will snap exactly to the pixel grid.
Text layers will also snap, and the anti-aliasing of text will be fixed no matter the position. With smaller text this may mean that letter distances might snap to a different pixel averaging, i.e.: kerning may vary based on position.
Placed image layers at 100% original pixel resolution remain unaffected by all these settings, but whenscaled and/or rotated, or distorted, or similar, the anti-aliasing setting will have an impact. Unfortunately, pixel snapping doesn't seem to affect these transformed images, which results in issues when pixel art, for example, is positioned to a decimal x and/or y position and anti-aliasing must be turned off: pixels will disappear in this case. Ideally images would always snap to pure non-decimal values when pixel snapping is turned on. The only viable solution in this last case is to manually adjust the x and y position to non-decimal ones.
System: Win10 64bit - i7 firstname.lastname@example.orgGhz, p6t Deluxe v1, 48gb (6x8gb RipjawsX), Nvidia GTX1080 8GB, Revodrive X2 240gb, e-mu 1820, 2XSamsung SA850 (2560*1440) and 1XHP2408H 1920*1200 portrait