I just discovered that the Sequential checkbox works inconsistently depending on where in the tree a project sits.
The desired behavior, which happens often but not always, is for the Sequential checkbox to cause the marked item to be queued for execution after its non-sequential siblings (after its “parallel/normal” siblings, that are not marked Sequential). All siblings marked sequential should then be activated automatically one at a time after all the non-sequential ones have been completed. This behavior seems to happen most of the time, which is good.
In some cases, I think mainly when you mark a top-level project or the sub-project listed first, the effect of setting a project to Sequential is that the project stays “active” but its children are treated as if all except the first one had been marked Sequential. This is how it works in most other apps, and which is what makes it impossible in those apps to choose any mix of parallel and sequential children. Either way, it should be consistent.
One thing that is getting increasingly apparent is the need for a consistent graphical indication in the outline of which items are “enabled” (parallel/active, whatchamacallit) and which ones are “disabled” (inactive/sequential, on hold). (And, sorry to trouble you, swear if you like, you are also going to need a third flavor for “indirectly disabled”, for an enabled item that has a disabled parent or grandparent etc and therefore cannot show up on the main lists yet. Without this third flavor it will be impossible to tell the difference between enabled and disabled items in a disabled project.)