You should configure a catch-all rule that specifies blank product category, tax type, tax code, unit of measure, and exempt reason fields. In this case, any transaction that specifies these items but doesn't match any other rule will be taxed at the rate specified by this rule.
Because this rule will match all transactions, where you place it in the list is important. If you place it at the end of the list, it will be applied to all transactions that do not match higher-ordered rules. If you place it higher in the list, rules following this catch-all rule will never be applied.