Signals of relative order zero in continuous time are common

The main difference between digital and analog systems is discrete or continuous

values

Every discrete part of our control system has to operate according to the same clock

A discrete system (transfer function) with poles of negative real part is unstable

Regardless of the plant discretization method, the controller can employ another one or be discrete from the start

The number of poles at zero in discrete time play exactly the same role as the integrators in continuous time

Every discrete part of our digital control system must satisfy the Nyquist sampling theorem but they do not need to use the same clock

Only one discrete system can approximate the operation of a continuous system

It is enough to sample at strictly more than twice the highest natural frequency of a system

A discrete system (transfer function) with poles of positive real part is unstable