from gen1, 2, 3 and 4? they have different speed but if I want to buy it then things get confused on SSD NVME.
basically I am asking about the form factor?
Here is the Wikipedia description on form factor and keying -- https://en.wikipedia.org/wiki/M.2#Form_factors_and_keying The length of the device and the type of key in the socket are the important things. Then the PCIe generation and number of PCIe lanes in the socket determine performance.