A Photoshop document‘s physical size, pixel density (DPI/PPI) and pixel dimensions are all linked.
Bitmap images contain bitmap data, which are a specific size (pixel dimensions). They also contain a DPI/PPI tag, which is usually a single value or pair of values (one for X, one for Y). The image’s physical size is calculated using the pixel dimensions and DPI/PPI. This is how some print design apps place images at “100%” optimal size, which will typically be a different physical size to viewing an image at 100% in Photoshop.
Please note that the questions below relate to changing a document’s DPI/PPI without changing the pixel dimensions of the image.
Given that DPI/PPI is just a single value contained within a Photoshop document, which — if any — features in Photoshop change depending on the document’s DPI/PPI?
Do gradients and gradient dithering look different with different DPI/PPI values?
Do other layer styles look different with different DPI/PPI values?
Sometimes metric measurements are used instead of inches, like pixels per centimetre.
In all my years of working with Photoshop, the only one that really stands out to me (aside from “print” and saving to other PPI-sensitive formats like PDF, obviously) is the type tool. But that’s primarily because the type tool always defaults to points (even if you pull up the type dialog and type in pixels, it’ll just convert it to points for you). Still, for a new Photoshop user who creates a 300 dpi image for the first time, they might be surprised that 12pt font suddenly looks huge on their screen.
Aside from that, I haven’t noticed PPI mattering much. After all, it doesn’t really make sense for Photoshop to make any of the other tools/features behave differently based on PPI resolution.
At the end of the day, a raster program has to render everything onto a pixel grid. PPI is just meta data for converting to physical units. There’s no way for it to affect gradients or dithering. Whatever PPI you use, it’s still 1 pixel per pixel.