A configuration can arise where one or more agents look at (query) another agent. Alternatively, an action might trigger a note to be moved, in turn triggering an $OnAdd and so on.
When using 'Update now' from the File menu, all agents are fired once (in $OutlineOrder sequence) as are any action events dependent on the update cycle. In a cascading scenario, it may thus be necessary to call 'Update now' several times to flush an action through to completion - especially if using linked agents where the order of dependence doesn't follow $OutlineOrder. For instance, if agent C looks at agent B which looks at agent A, a change in A will need 2 updates to flush through to C.
Therefore, if using documents with cascading effects, be aware that a single forced update may not always 'complete' all actions as the user might intuit would occur from a single invocation.
Forced invocation of updates runs all running agents, i.e. regardless of priority setting unless the agent is turned off.