From my understanding the auto hold button (the buttton at the back) you leave on unless you want to use the parking button (the switch at the front) as if it was a normal handbrake. Which I guess you would if you didn't trust auto hold or just preferred doing it manually. Interestingly it appeared the reviewer from the car obsession videos linked in another thread seemed to be using it like this, flicking the button every red traffic light.
On the other hand, if auto hold is on the car will automatically apply the handbrake when you brake to a standstill and then come off the brake, without needing you to manually activate the front switch. Only time you use that is when you stop for parking just to be safe before you step out the car! My only thought is whether autohold works if you have your foot on the clutch in first gear at lights ready to go, usually your foot would be on the brake in this situation so I guess it's not needed, but im presuming it wouldn't work. A green P acts as a visual clue in the dash when autohold is activated anyway and will disappear as you bring clutch up to pull away/touch the accelerator in an automatic. I think it's a red P when you pull the parking brake button yourself.
Also hill hold is a different function, my old car had that which didn't have an electronic brake. Basically if it detects you are starting to roll backwards it will apply the brake for a few seconds to let you sort yourself out and pull away smoothly! I believe the Ateca with its more advanced system will hold you indefinitely. Very useful 99% of the time, but once in my old car I found myself behind a learner driver on a hill, they tried to pull away but alarmingly started going backwards. I put my foot on clutch expecting my car to roll back to miss him and nothing happened, hill start had engaged and stopped me rolling back! Meanwhile learner driver was still rolling towards me. Luckily the instructor got on his dual controls and stopped their car. Heart in mouth moment though and one instance I wish I had full control!