How does setpose work in rivz panel

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Does rivz just send out the pose on the topic /initialpose (that can be overridden in properties panel). Or does it not only do this but also uses the set pose internally not though the topic. I was trying to override the topic but it almost looks like even though I can override it on a new topic rviz still somehow uses it before sending it out rather then being a subscriber. Where can I find the code responsible for initialpose in rviz?