As mentioned already
Sounds on the +Drive are permanent records of sounds, the only way to alter those is to overwrite one deliberately
These can be used to populate kits whenever you need and to populate the sound pool whenever you need
Think of the +Drive as being something outside the normal working area, you pull things in from it, and importantly, you save things out to it (i.e. sounds you create that you want to have and use later elsewhere)
Sounds loaded into a Kit become immediately divorced from the source, these are now only part of that kit and that kit may be linked to any number of patterns, so changes made to the kit whilst working on pattern x will be reflected in pattern y (linked to same kit) - so you have to keep in mind that changes can affect other work in this way - every pattern is linked to a Kit (Kits can exist unused)
Sound pool sounds are best thought of as a source for sound locks, imho it is best to use sounds from the +Drive to populate the sound pool so that those sounds are always available to all projects (you can create new sounds and keep them solely in the pool if you wish) … sound locks need to work with sound pool sounds, so it may be advantageous to use drum sounds in the sound pool as these would not typically be used in isolation as the main track sound, but you’d use one of many to make up a drumkit … sound pool sounds can be any type of sounds, but if you are using the pool to load only track sounds as opposed to the +Drive then you gain no advantage from the speed with which sounds can be extracted from the pool
you can record realtime sound-pool sound locks and you can assign them in step record mode too + multi map mode
Oh, and welcome 