Difference between Topic DefaultMessageTimeToLive and Subscription DefaultMessageTimeToLive on Azure Service Bus

Both a Topic on the Azure Service Bus and an associated Subscription expose the DefaultMessageTimeToLive property; initialised like so:

if (!NamespaceManager.TopicExists(TopicName))
{
    NamespaceManager.CreateTopic(
        new TopicDescription(TopicName)
            {
                MaxSizeInMegabytes = 5120,
                DefaultMessageTimeToLive = TimeSpan.FromDays(14)
            });
}

if (!NamespaceManager.SubscriptionExists(TopicName, SubscriptionName))
{
    NamespaceManager.CreateSubscription(
        new SubscriptionDescription(TopicName, SubscriptionName)
            {
                LockDuration = TimeSpan.FromMinutes(5),
                DefaultMessageTimeToLive = TimeSpan.FromDays(7),
                EnableDeadLetteringOnMessageExpiration = true
            });
}

What is the difference between the two and what is the purpose of having two TTL settings? Furthermore; if a message expires on the Topic what happens to it?