It's unclear in which context you are evaluating these expressions. Since set analysis is evaluated once per chart, not per dimension line, I assume you are using this expression in a text box (or in a chart, but with dimensions SECTION and SECTION_Comp limited to single possible values).
A small sample application together with the expected result might help us to understand what you are trying to do.
Just guessing around, maybe something like this could work:
= (
if(SECTION=SECTION_Comp,0,count({<SECTION=p(SECTION_Comp), [BILLNO]=p([BILLNO])>}{<@_Date=
{">=$(vMonthStartCY)<=$(vMonthEndCY)"},@_Month=,@_FinQuarter=,@_FinFullYear=>}DISTINCT [BILLNO]))
)
/
count({<@_Date={">=$(vMonthStartCY)<=$(vMonthEndCY)"},@_Month=,@_FinQuarter=,@_FinFullYear=>}DISTINCT [BILLNO])
)
or
= (
if(SECTION=SECTION_Comp,0,count({<SECTION=p(SECTION_Comp), [BILLNO]=p([BILLNO])>}{<@_Date=
{">=$(vMonthStartCY)<=$(vMonthEndCY)"},@_Month=,@_FinQuarter=,@_FinFullYear=>}DISTINCT [BILLNO]))
)
/
count( TOTAL<SECTION>
{<@_Date={">=$(vMonthStartCY)<=$(vMonthEndCY)"},@_Month=,@_FinQuarter=,@_FinFullYear=>}DISTINCT [BILLNO])
)
But too much guessing...
