The choice of zero-halogen (ZH) vs not is largely one for the regulations. Typically, if you are governed by IEC codes, the European community is more in favour of ZH cables, while in North America there is less use of these cables (unless you are US military).
In any event, IEC 60754 has the test for measuring halogen content and I believe the max % concentration is 0.5%. In the US, MIL C-24643 governs and the limit is 0.2%.
As to why you would specify ZH or not depends on whether you believe that burning of these types of cables would help or hinder during a fire. There are two groups lined up opposing each other and one group says the use of PVC compounds are OK as long as the fire does not propogate, while the other group says that the use of halogen cables creates too much toxicity and smoke.
