3-piece setup is battery + atomizer heating part + cartridge aka cart (just plastic and stuffing)
(alternately for 510-T or Ego-T, battery + tank atomizer + tank)
2-piece setup is battery + cartomizer aka carto (a 2-piece disposable combined cart with internal atomizer)
Most people use cartomizers for their favorite suitable juices. Some high VG juices are too thick for many cartomizers or at the very least more difficult to use in cartomizers. I use thinner juices, 20% or less VG, in cartos. I also avoid juices that drop a lot of flavoring sediment because they clog up cartomizers
The most cartomizers are available for 510 and KR808D-1 threaded devices. Most of the 510 ones are blank (you fill them). The KR808D-1 model (since it is a 2-piece sold with cartos) has plenty of prefilled and blank cartos.
Cartridges have almost unlimited lives - you can wash/dry and restuff them. The atomizer is the part that needs to be kept clean and can get weak or burn out.
Cartomizers have limited lives. You cannot replace the atomizer in them, you cannot replace the stuffing. I can get a week or more use out of one carto (2 weeks or more if I alternate using a pair). then I start new cartos. Some people succeed at cleaning cartomizers to get some additional like out of them but many may never vape as well as when new. Most don't bother cleaning them. They cost $2-3 apiece for the simple stuffing cartomizers and higher for more exotic types and reservoir types.
benefits of cartos - your flavors are totally separate and not sharing an atomizer. They hold more liquid than cartridges and work more consistently. The stuffing kind will not suddenly dump 1/2 of their
juice like a cartridge can. every cartomizer has a fresh clean atomizer.