The relay is a mystery to me.
The tiny copper line to the pressure switch goes to the same place as a similar line that goes to the coils. I guess that's freon.
If we left it running continuously, the pressure switch would turn it off. That made me nervous, so we measured water temperature at a faucet and turned it off around 120º. Bit of an inconvenience, but we aren't people who need everything done for us.
It is annoying if the first showerer doesn't remember to turn on the heat pump. Only a short delay every summer until we get into the swing of things.
Dehumidification in the house isn't automatic either. We measure it constantly, turn on dehumidifier(s) as needed. Had one die this year, grabbed one of my spares a friend had given me. He'd said it worked great, until the control failed. Bought another control which failed soon after. He gave up and bought a different brand.
I wired around the control, bypassing it entirely. Now it's on or off, only on high. Works perfectly for us, my wife's delighted. Far stronger than the one that died.
Heat pump is similar.