It's the threading and gender of the battery. It determines what parts you can use without an adapter (within the constraints of also choosing appropriate ohms if you have a slim battery device). If you buy some random rebrander model it may have an odd threading and there may not even be adapters.
There are a few threadings people like for slim ecigs but mainly 510 is the most popular 3-piece and KR808D-1 is the most popular 2-piece. Most people use the 510 threaded devices as a 2-piece with cartomizers but since it is a 3-piece as-sold, there are few prefilled cartomizers. The KR808D-1 has a lot of prefilled cartomizers. The fat battery models and mods are predominantly 510-threaded, a much smaller number are 901 threaded and can use many KR808D-1 cartos with no adapter, and a small number of mods are 801 threaded.
If you look at my signature below
510 threading: my 510, Riva 510 (fat battery model), 3.7v mod box
KR808D-1 threading: KR808D-1 (mine are from Vapor4Life and SmokelessImage)
I also own a 510 to 808 adapter to put my prefilled KR8 cartos on the 3.7v mod box Riva 510 (it's the enw 3.7v model so it has enough voltage to do them justice). KR808D-1 is a real 3.7v slim ecig (more voltage than 510). And Boge is the most popular brand of blank cartomizer for 510 threaded devices. A cartomizer is a combined disposable cartridge and atomizer (I can get a week of use out of each with thin clear juices)
And BTW - I strongly recommend manual batteries. Most people here use and recommend them. Easier to drag, easier to get a good hit, harder to damage with juice leaks.
NOTE - If you happen to buy a slim 510 I recommend Joye brand only for good quality and to avoid a particular reverse polarity clone. For the fat battery Riva 510 I recommend Liberty-Flights.com which has the kits which are the new 3.7v model from the correct manufacturer that makes that model. 100mah is about 1 hour of moderate vaping. Joye 510 180mah. Joye 510 mega xl 280mah. KR808D-1 is 180 or 280 at Vapor4Life and 220 or 320 (manuals are mostly sealed and have a standard size button) for SmokelessImage Volt (manuals are sealed, only available in some colors and have a particularly tiny button). I recommend the larger of those two sizes to make 120mm ecig. Joye Ego and Riva 510 (a popular Ego clone) are 650mah and up with some big batteries as high as 1100 or 1300mah