Why is it called 510-thread Cartridge ?

The 510 connector get its name from the length of the connector and the amount of threads it has for it to 

make a connection. It runs 5mm long and 5 threads is on the male connection, which is on the atomizer, 

and the other 5 is on the female connection, which is on the battery or mod.

The simple answer is that any cartridge with standard 510 thread will fit on any standard 510 threaded battery. 

99% of batteries and cartridges are interchangeable with one another.

510 thread vapes are the standard in vaping. The “510” is the connection between the atomizer and the battery

 device—it's how you screw on the tank.