When an IP packet is sent, an IP header field called Time To Live (TTL) is set to a value between zero and 255. Every time a router forwards the packet, it decrements the TTL field in the packet header, and if the value reaches zero, the packet is dropped. The IP specification also states that TTL should be decremented if a packet is queued for more than a certain amount of time, but this is rarely implemented these days. With unicast, TTL is normally set to a fixed value by the sending host (64 and 255 are commonly used) and is intended to prevent packets looping forever, and also forms a part of the formal proof that the TCP close semantics are safe.
With IP multicast, TTL can be used to constrain how far a multicast packet can travel across the MBone by carefully choosing the value put into packets as they are sent. However, as the relationship between hop-count and suitable scope regions is poor at best, the basic TTL mechanism is supplemented by configured thresholds on multicast tunnels and multicast-capable links. Where such a threshold is configured, the router will decrement the TTL, as with unicast packets, but then will drop the packet if the TTL is less than the configured threshold. When these thresholds are chosen consistently at all of the borders to a region, they allow a host within that region to send traffic with a TTL less than the threshold, and to know that the traffic will not escape that region.
An example of this is the multicast tunnels and links to and from Europe, which are all configured with a TTL threshold of 64. Any site within Europe that wishes to send traffic that does not escape Europe can send with a TTL of less than 64 and be sure that their traffic does not escape.
However, there are also likely to be thresholds configured within a particular scope zone - for example most European countries use a threshold of 48 on international links within Europe, and as TTL is still decremented each time the packet is forwarded, it is good practice to send European traffic with a TTL of 63, which allows the packet to travel 15 hops before it would fail to cross a European international link.