This globally enables, disables, or defers to per-route settings for transmit-side IP header compression. The perRoute(2) option matters here because header compression benefits vary a lot by traffic type, since VoIP with lots of small packets gains a lot while large file transfers gain little, so per-route control lets bandwidth savings be targeted where they actually help. Setting this to perRoute(2) and enabling cdmipIpRouteHdrComp only on a route carrying VoIP traffic would compress those small packet headers without adding processing overhead to a bulk-data route that would not benefit as much.