Asking to clarify is good. We're here to teach as much as help, possibly more so the former.
That's the fun part. If we add some Watts through one wall, the domain will heat up. At the other side we'd typically set a temperature. The flux then pushes against that temperature so the bulk domain will warm up to suit the set flux and temperature. Poorly set boundary conditions may then give very silly temperatures.
A flux in and a flux out will typically give a result of infinity or absolute zero for temperature. This usually is enough to cause divergence or lots of limit warnings.
So, in most cases we'd have some boundaries as heat flux and some as temperature (that may be a convection boundary, but it still gives us a temperature anchor).
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