What does SR and LR stand for? What difference does it make?

Regarding a cartomizer:

SR stands for Standard Resistance: (about 2.7 - 3.2 ohms)

LR stands for Low Resistance: (about 2.0 ohms - 2.6 ohms)

Ohm's are an electrical measure of resistance. The higher the resistance, the higher the ohm level will be. The lower the resistance, the lower the ohm level will be. Going along with the ohm ratings- the higher the ohm level is the lower you will need to set your voltage/wattage at. The lower the ohm level, the higher you will be able to set your voltage/wattage at in comparison. 

For more great information on Cartomizers feel free to reference these great links to our blog: 

http://blog.mtbakervapor.com/510-cartomizers-the-basics-every-vaper-should-know/

http://blog.mtbakervapor.com/an-introduction-to-cartomizers/