To support parallelism, specifically the ability to work on two or more activities at the same time, we need to employ separate tickets, one for each branch of parallelism. This introduces the two additional shapes to achieve.
A Join/Split Gateway shape to define that all transitions are followed to activities, as opposed to choices where no Join/Split Gateway is used. To underline the fact that a separate ticket is required the Parallel Activity shape is used to achieve this. However, one of the activities in parallel must be a standard activity. This can be referred to as the 'parent' ticket, and the parallel ticket(s) are referred to as either 'child' or 'sub-tickets'.
This is the basic parallel process pattern. W will transition to X, with Y created in parallel.
At this point, Y is said to be blocking X from transitioning Z. When the sub-ticket representing Y is solved, it unblocks to allow a transition between X and Z.