It's a "Local Agreement" with the Inland Revenue. The allowances (or sector/duty pay) are to recompense us for the additional costs which we incurr during the course of our work. Like buying sarnies or water (if you work for some operators, or gin if you work for others) whilst at work. And then consider that same have to slip a few days - it is often very expensive where crew stop!. If we were taxed on these, then we would keep asking for these additional expenses to be taken into account for tax purposes and that would cause too much effort (for everyone!). Therefore, not all allowances are taxable.