I'm not quite sure if this is what you're looking for, but anyway, try to help you!
X-Axis is for the lengh of the repeating sample
Y-Axis is for the pitch of the repeating sample (tone high)
The sample its self repeats f.e. in basstime, if you choose for sidechain band f.e. bass.
Secound, the use:
I mainly use it by doing some vocal looping or with melodie strips! F.e. House vocals, you can sample them and with some effects like f.e. the echo, you can give your mix more "space"!
Or if you sample at a beat, you can keep it "rolling" in the dropdown sections of a track...
You see, I use it more as an ambience effect than an exactly sample or roll!