The ultra short throw projectors mount only inches from the presentation (screen) wall, ideal for interactive presentations as the teacher or student can avoid being blinded by the projector light. Very short throw projectors tend to be significantly less, still have the advantage of mounting to the “screen” wall, rather than the ceiling (less installation cost), but aren’t as good as keeping the projection light out of the user’s eyes.
While very short throw projectors have been coming more and more popular in general, with or without interactive features, the ultra short throw projectors are mostly reserved for interactivity, as doing better there, helps justify the higher costs. Very short throw projectors usually have their lenses 3 to 4 feet from the screen, which means they can mount on a telescoping wall mount.
Image depicts how ultra short-throw projectors prevent presenters from being blinded, due to how close they can be placed to the screen surface. Image from Epson.