Hi!
Since TPD regulations kicked a few years ago in you can't buy eliquid containing nicotine in bottles larger than 10ml, or higher than 20mg in strength.
This is fine for many vapers, but many also prefer larger bottles so to get around the regulations larger quantities are sold nicotine free with space to add your own nic shots. Its usually 50ml + 1x18mg 10ml nic shot to make 60ml at 3mg, or 100ml + 2x18mg 10ml nic shots to make 120ml at 3mg. If you want 6mg then double up the nic shots.
There's also nic salts now too, which are stronger but smoother than regular freebase nicotine and enter the bloodstream quicker for a more cig-like hit. These are sold in 10ml bottles, usually 10mg or 20mg strength. These are better suited to pod devices and Mouth To Lung tanks, rather than sub-ohm tanks and Direct To Lung tanks.