My understanding is that if a candidate quits or suspends his campaign, the delegates are cut loose, so they would be unbound for the first ballot. Rubio is challenging that this time around by requesting each state to keep his delegates bound. I think if he later comes out and endorses Cruz or Trump and encouraged his delegates to honor his endorsement, many would likely follow suit. But, I don’t believe it’s required. Kasich or Rubio might end up being the kingmaker!
I think suspending is not considered quitting. It means they are still a candidate but that they aren’t campaigning.
I think they have to drop out, quit.
The Reson Kasich is staying in is because under the State rules his delegates go to the runner up if he drops out. That would be Trump, they are not unbound.