zadar

first number (19) is master cylinder bore. Second number (18, 20) is leverage ratio, distance from levers pivot to where plunger is attached on lever. 18 is shorter than 20 (closer to levers pivot) so you will have to pull lever more in order to push plunger same distance you would with 20. 18 feels softer and you apply brake power by pulling more while 20 feels harder and you have to squeeze it harder to get braking power.
There are also new ones which are adjustable for leverage, they simply move plunger point along lever by turning nub.
If you buying none adjustable it is better to get 19x18 because if you don't like lever travel you just buy 20 lever and you get 19x20. It can work other way around also but it puts plunger on more angle.