what is 'fpt.domvar_deff1' in fluid particle tracing module

Hello everyone
I want to calculate the dielectrophoresis force by the fluid particle tracing module(fpt).
But the equation puzzled me:
2*fpt.rp^3*pi*fpt.er*epsilon0_const*
(fpt.erp-fpt.er)*d(model.input.E1^2+model.input.E2^2+model.input.E3^2,y)*fpt.domvar_deff1/(fpt.erp+2*fpt.er)

I don't know what is the 'fpt.domvar_deff1' .I cannot find its defination.

Any one can help me ?
Thanks!