Some nodes, like the ones shown below, have input ports that block an event. This means the node will execute, but the event associated with that data won’t travel through any output ports. Event blocking is useful when you want part of your composition to execute in response to events from one trigger port but not events from another trigger port, or when you’re creating a feedback loop.
Ports that always block events have a solid semi-circle (like the URL port above) or a solid chevron (like the Start port above). This is called an event wall. The node must receive an event from another port without an event wall for the results of the node’s execution to be available to other nodes.
The event wall is visually placed inside the node to indicate that the event gets blocked inside the node (as it executes) — rather than getting blocked before it reaches the node.
Ports that sometimes block events have a broken semi-circle (like the Which port above) or a broken chevron (like the Time port above). This is called an event door. Event doors are useful when you want to take events from a trigger port and filter some of them out or route them to different parts of the composition. For example, in the Select Output node, the value at the Which port will determine whether the data-and-event coming into the In port will be transmitted to the Option 1 port or the Option 2 port.
The manual section How events travel through a composition has more information on how events move through a composition.