What would a circuit labeled "Heat Trace" be used for?
'Heat Trace' is a subject of replenishing the rate of 'Heat Loss' taking place from the hot surfaces to be kept heated for the specific needs at required temperatures. These heat loss surfaces are generally 'thermally insulated' to reduce the rate of heat loss taking place, and this prevents around 80% to 85% heat loss rate occurring. 'Heat Trace' function is to replenish the 15% to 20% rate of heat loss taking place; the effective drop in temperature from the heated thermally insulated and 'heat traced' surface is zero. Applications for 'heat trace' in Commercial / residential buildings would include, but not limited to are: Prevent water pipe from freezing in sub-zero climates; to maintain domestic hot water pipes at above 60 deg.C to prevent formation of dangerous bacteria such as Legionella; in a centralized hot water distribution piping network to maintain water temperature during no-flow condition such as during night time, ensuring instant or quick flow of hot water to taps, and prevent wastage of cold water flowing through hot water taps; comfort floor heating during winters; heat trace gas pipelines to prevent condensation of gas in the lines. It is also used prevent condensation taking place at locations such as freezer doors to prevent ice formation.

For industrial application, heat trace of pipelines, vessels and tanks is carried out for handling difficult fluids having high viscosity, high pour point temperatures to prevent solidification and plugging of lines. Also for prevention of condensation.

Heat Trace is carried out by means of Electrical Heating cables of low watt density, by Steam and hot circulating fluids. For commercial applications, Electric Heat Trace is generally carried out. As the electrical heating cable or the Steam tubes or heated fluid tubes are fixed to the Pipe surface and runs along its length, like tracing the cable or tube along the pipe length has been termed as 'heat tracing' and hence called 'heat race'.

If you search on the Net for 'Electric Heat Tracing', you would find lots of information from many of the manufacturers of heat Trace cables.