Multicast:
#9. Multicast Rate:the multicast rate is the rate by which your Base Station sends a single message to a select group of computers on your network. In simple terms, if you use a lot of streaming content, you're better off setting a higher multicast rate to improve streaming performance but this has the effect of reducing the effective range of your network. If your usage patterns doesn't require high loads on your WiFi network but instead longer range, lowering the muti cast rate extends your WiFi coverage.
In short, the higher the multi-cast setting (under wireless options in the airport base station management software) the lesser the range f the network but the better the performance, especially under streaming situations.