Weak sequencing reduces to a parallel merge when the operands are on different sets of participants. Weak sequencing reduces to strict sequencing when the operands work on the same participant.

I recommend using the term “parallel” when the simultaneous execution is assured or expected, and to use the term “concurrent” when it is uncertain or irrelevant if simultaneous execution will be employed.

Concurrency is about dealing with lots of things at once. Parallelism is about doing lots of things at once.

A system is said to be concurrent if it can support two or more actions in progress at the same time. A system is said to be parallel if it can support two or more actions executing simultaneously.

Parallelism means that an application splits its tasks up into smaller subtasks which can be processed in parallel, for instance on multiple CPUs at the exact same time.

Concurrency means executing multiple tasks at the same time but not necessarily simultaneously.