The term 'relativistic mass' is superseded and many answers in this site often state that the term mass ought to be used ony for rest(/invariant) mass.
John Rennie explains that :
... but you need to be clear that the relativistic mass is a computational device and it does not mean the mass is changing. Nothing happens to the gold. If I am sitting by the gold while you speed off in your spaceship I will see nothing happen to it.
But he adds that:
the key point. When we heat the gold all observers, no matter what their speed, will agree that the gold has changed because we all see it absorbing the light from the IR lamp
So, it is intended that the mass of the gold has indeed changed, is that so? If that is the case, besides rest mass there is something more, but John Rennie does not specify how the gold has changed and if we are allowed to call the increased entity mass in which case maybe thermal mass is the correct term? If it is mass, in what way is it different from the mass of an electron?
Do you happen to know if there are any theories explaining what really is going on: how does the gold change and 'mass' increases?