ISAF is the International Security Assistance Force in Afghanistan.
A "zap code" is a personal identifier used in the military. Its form varies from unit to unit. In the U.S. military it is longer and consists of both letters and numbers. I don't know about British or ISAF formats, but if this comes from a work of fiction (as I suspect, though you didn't say) it may not be 100 percent accurate in all its details.