Search in the Forum, I think this has been discussed before quite a few times.
In general, this is a balancing factor of your Response spectrum base shear to the static base shear. For this, you should know your static base shear as per your Code of Practice. This can be calculated by selecting relevant Code and using Auto Seismic option in load cases menu in ETABS. Depending upon your framing system different Code allows different %age of base shear to be balanced. Normally, It is 90% of the static (Codec) base shear.
IR