What is pi?

Hello,
A friend sent me this portion of his code and I'm trying to understand what the parameter 'pi' is in the RDHFHet model:
S.treat.spec.t.gap=list(formula=~treat*spec*t.gap)
p.c.diff.treat.spec.session=list(p=list(formula=~treat*spec*session,share=FALSE),c=list(formula=~treat*spec*session))
pi.treat.spec.session=list(formula=~treat*spec*session)
Thanks in advance,
Sergey
A friend sent me this portion of his code and I'm trying to understand what the parameter 'pi' is in the RDHFHet model:
S.treat.spec.t.gap=list(formula=~treat*spec*t.gap)
p.c.diff.treat.spec.session=list(p=list(formula=~treat*spec*session,share=FALSE),c=list(formula=~treat*spec*session))
pi.treat.spec.session=list(formula=~treat*spec*session)
Thanks in advance,
Sergey