How can I tell what the default behavior for a cancelable event is?

For example, I read somewhere that the TextEvent.TEXT_INPUT has a default behavior and that default behavior includes adding a text character associated with the key that was pressed to a TextInput. That makes perfect sense.

But if I hadn't read that, how would I know what the default behavior is? Other than guessing. In this case, it's probably obvious. But in other situations, it might not be.

For example, in the docs, look at DataGridEvent.HEADER_RELEASE's cancelable property. It says:

cancelable: true

so, there appears to be a "default behavior" associated with a DataGridEvent.HEADER_RELEASE event. But what is it? And why would I cancel it if I'm not really sure what it is? :)

thanks.