What is a "related target" for an event in ExtJS and/or DHTML?

When using ExtJS I've noticed a property "Related Target" for an event. Googling also reveals that this term is used in other DHTML related contexts, not only ExtJS. So what is this "related target" and how does it differ from the开发者_StackOverflow社区 normal target (which is another property of the event object)? On this subject google revealed nothing.

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It refers to the element the mouse goes to, or comes from. The only standard events it applies to are mouseout and mouseover.

See QuirksMode and MDC.