I actually ended up wondering the exact same thing with my character concept - he's typically using an arachnoid synthmorp, but is also carrying around a dragonfly as a sort of an utility option. The 'fly morph is equipped with a puppet sock just for the purpose of jamming in it if necessarily.. in any case, the difference between sleeving and jamming I understoodt is the timespan. When you sleeve, you sort of intend to *stay* there for a while, while jamming is at best temporary.. and for a good reason.

First, you need to be sleeved into something to operate - even into an infomorph.
Second, the jamming thing requires a constant connection between your sleeved form and the jammed unit. Assume you've made a character who's an infomorph in a computer, operating in the 'real' world by jamming socked shells. Yes, this means that as long as no-one touches the computer you're essentially immortal, the loss of a body won't slow you down.. but by the flipside if something disrupts your comms you've got no mobile body *to* operate. At least this is how I understoodt it to work.

In my example, my character can jam into the dragonfly when necessary to scout around, but losing the 'fly won't impair him overmuch - he'll just find himself in control of the arachnid again. Granted, if the 'fly was destroyed without him dumping the connection, he'll suffer 1d10 points of stress..