Take a look at excel's help for =iserr() and =iserror().
Stolen from the help for =iserr(value)
Value refers to any error value except #N/A.
And for =iserror(value)
Value refers to any error value
(#N/A, #VALUE!, #REF!, #DIV/0!, #NUM!, #NAME?, or #NULL!).
So your formula looks like this:
=IF(ISERR(RC[-2]),0,IF(ISNA(RC[-2]),0,IF(EXACT(RC[-6],RC[-1]),1,
If it's any error except #n/a (iserr()), you return a 0. Then right after that,
you check to see if it's a #n/a (with the =isna() function. If it is #n/a, you
still return a 0.
So why not just use one error check that gets both: =iserror()?