Subroutine usproj.f90

[Damien Charreron]

Hello!
I have try to take a look to the subroutine usproj.f90 in order to write the mass flow and the thermal flow on all my boundary condition.
I begin to understand how it works, however I am stuck with the following equation: 
 
    xfluxf =    surfbn(ifac) * dt(iel) * xcp * (xvsl+visct/sigmas(iscal))/distbr * (  coefa(ifac,iclvar)+(coefb(ifac,iclvar)-1.d0) * treco(ifac)) -   flumab * dt(iel) * xcp * (  coefa(ifac,iclvar)+ coefb(ifac,iclvar) * treco(ifac))
    xbilen = xbilen + xfluxf
 
Could somebody help me to translate this into mathematical language please?
Also how could I write the mass flow for each boundary condition? I tried "write flumab"... but doesn't work.
 
Best regards!
 
Damien CHARRERON