Machine Legs / Feet
The ergonomic operational hight of an espresso machine will vary or bench and operator height. Commercially council regulations may determine the clearance required between the bench and machine for cleanliness, if so this is generally 100ml. And operationally the machine needs to be level (actually a level should always be checked on machine instal otherwise uneven flow at the group head may occur, and as such adjustable feet can be used to level the machine (or some folded cardboard like seen on many installs...)
The main issue to consider when selecting machine legs/feet is to make sure the thread size will fit the machine. Commonly this is a 10mm thread however some machines have now fitted unique threads to force the user to buy their specific feet/legs at inflated prices.