Sometimes, a Due transition (transition with a clock over it) can help keep a process instance moving along. Think of the Milestone pattern as a way to set an SLA for an individual activity. If an instance sits too long in the activity, it is automatically transitioned through the due transition to the next activity.
In this example, the purchasing agent will award the contract to the lowest bidder if either one of two things occurs:
1. If not all the suppliers have placed their bids, the due transition times out after one week or
2. If the same number of bids reach the Join activity that were spawned in the upstream Split-N activity.
Due transitions are sometimes used when Join, synchronous sub-process invocations and Notification Wait activities might cause bottlenecks in processes.