What does CSTR means on an Airbus FCU and what happens when CSTR PB is pressed?

Question

I am searching for:

1. The use of CSTR PB on Airbus FCU &
2. During what phase is it used?

Thanks

Answer 1

It's a Constraints button that When pressed shows altitude or speed constraints at a certain waypoint.

It would normally be used on departure or arrival from or into an airport when there are many constraints that need to be met.