Ableton's tutorial on Granulator III covers the two alternative playback modes available beyond the default Classic mode: Loop and Cloud. Each mode changes how grains are played back and which controls become active.
Loop mode behaves like a conventional sampler, repeating a selected section of the loaded sample. The Shape knob controls the crossfade at the loop point, and when Grain Size is selected, a reverse control appears that sets the probability of any individual grain playing in reverse.
Cloud mode layers up to 20 grains simultaneously to produce thick, textured sounds. Shape is not available here, but a density control becomes accessible when Grain Size is selected, letting you set exactly how many grains are active at once.