Meta question first: I'm confused. Is there no general sub-forum to ask generic running related questions? This is not really specific to my 735XT and I have the feeling that people might be able to help me out but don't bother looking into every watch section of this forum..
Anyway, I hope someone can help me out, I wasn't able to find anything online 
1) Why am I supposed to run my 5k race next week in HR zone 3 - it doesn't make any sense, especially since there were speed trainings where I had to maintain zone 4 for 25 or 30 minutes. I'm not even planning on being on the road that long..
2) If it says "race tempo" for a session - what is that exactly? Am I supposed to run slightly below threshold level or even faster? And at what distance should I start out in zone 3 - should I even? (see question 1)
3) If my training sessions never dip into zone 5, how do I know my threshold is not moving upwards slightly? How would my watch be able to detect that?
4) Are HR zones automatically updated each time my threshold is re-estimated?