How to save the rest of us time on questions that someone who really knows what they're talking about has taken the time to explain:
1) Find out what you are
really trying to do. (In your case, I suppose you want a projectile to emit particles.)
2)
(and you get
Muffinmix's awesome guide to what you are trying to do)
3) Read until you understand. (Yeah, stuff about projectiles is after a bunch of other information, but you should still read it in order so you understand what he is talking about in the projectiles section.)
4) Think about how to accomplish what you wanted to do now that you know how. (What you originally wanted to doesn't really make sense because a projectile kind-of acts as an emitter node with its trail emitter.)
5)
(Make your cannonball that leaves giant green lightning bolts in its wake or whatever.)
