SCR, refers to you Surface Consuption Rate.  This is the average amount of air you use at the surface, and is used by the system to calculate you air consumption.  Your air consumption is an important parameter, it will allow you to plan you dives with more confidnce as well as ensuring you have a compatible buddy (ideally you should both have similar air consumption).

HOW TO CALCULATE YOUR SCR

METRIC METHOD : - 

Air Used (litres) = (SP - EP) x TWP
                               TWP 

SP = Starting Pressure (Bar)
EP = Ending Pressure (Bar)
TWP = Tank Working Pressure (Bar)

Surface Minutes (mins) = BT x AP 

BT = Bottom Time (mins)
AP = Absolute Pressure (ATM)

SCR (l/min) =   AU  
                           SM

AU = Air Used (litres)
SM = Surface Minutes (mins)