There are several workflow elements which can affect the choice of flow path (or paths) through a workflow at runtime. These include:
These elements always come in pairs. This is because they demarcate areas where the process should exhibit a specific behavior (one related to the flow of control). These areas are normally referred to as 'blocks', because they have a specific start-point that must have a corresponding end-point.
Consider a process with a structure where all paths emerging from a Choice Split (guaranteed to only follow one outbound path) all converge at a Parallel Join (which will wait until all inbound paths complete before executing the next activity). In this case, the process is guaranteed to stall at the Parallel Join. This is an example of a problem with the block-structure that can be caught by validations before a process is even executed.