Here is the C++ method definition:
Code:
void Unit::SetVisible(bool x)
{
if (!x)
m_serverSideVisibility.SetValue(SERVERSIDE_VISIBILITY_GM, SEC_GAMEMASTER);
else
m_serverSideVisibility.SetValue(SERVERSIDE_VISIBILITY_GM, SEC_PLAYER);
UpdateObjectVisibility();
}
So if x is true, it is visible to players etc
If x is false, it is visible to GMs only, so basically its just invisible.
It seems to be used a lot in scripts, however it is not enabled at the moment by default in Eluna due to it being TC only function and the same has not been found yet on mangos.