I am not familiar with the term but I would think that it is the gap between high and low voltage circuitry.
High voltage being hot, low voltage being cold.
Unless it is something to do with the manufacturing process itself.
I would go with voltages and protection, SELV, IMO it is a reference to creepage and clearance distances. For 99.99% of PCBs the temp range they work at will not affect conductivity that much that it becomes a concern.