You choose which size of needle head you need, 

1-2 can pick up to 1-2 strands of hair. 

2-3 can pick up to 2-3 strands of hair, 

3-4 can pick up to 3-4 strands of hair, the needles look pretty same to me! but they are in categories.

The needle itself has been magnified in the picture for a better view.