The wherecondition parameter has to resolve at execution time to a string that is a valid limitation on the query that underlies the form. As long as whatever you code fits that description, then you're good. For instance, if you had two control on the form that the VBA was behind, and Control1 had a value of "FirstName" and Control2 had a value of "George", then a parameter
Code:
"( " & Me.Control1 & " = '" & Me.Control2 & "')"
would resolve to
Code:
"( Firstname = 'George')"
which would be a valid limit if there was a field Firstname in the query for the form.
I've forgotten whether you get nothing or everything if the filter is not valid - I think everything.