What is the terminal labelled "U" in some autotransformers?
Sentera's website shows a connection diagram for a similar controller:



Figure 1. Source: Sentera.


  Pe for ground, L-N are my input, N1-L are unregulated output (same as L-N) and U-N are the output to my motor.


N1-L should read N-L1. Yes, unregulated output but on when running so it is switched.


  Q1: What is the U terminal? What does it stand for?


'U' is commonly used in Europe to mean 'voltage'. It makes sense when you think that we use 'I' for current and avoids confusion between 'voltage' and 'volts'. U1 and U2 are the fan terminals and the naming suggests that neither of them is at neutral potential so the triac is likely to be in the neutral line.


  Q2: Do I connect my motor's ground to the common Pe? I'd have imagined that there would be a dedicated terminal for the ground near the output to the motor


Yes. It appears that they have been a bit mean with the terminals.