The situation where a room is heated as a result of solar radiation coming in through the windows is a heat interaction since it is due to the temperature difference between the sun and the room that caused the heat.
By a virtue of a temperature difference due to thermal interactions, energy in the form of heat is transferred between a subsystem and its surroundings. Mass flow are the main participants of thermal interactions. However, this conversation about thermal interactions is confined to close subsystems only, i.e. zero mass flow subsystems opposite their boundary.
Radiation and conduction are the two difference ways where heat can be transferred thru a subsystem boundary.