They are used to determine airspace, obstacle clearance and visibility requirements for instrument approaches.
[1]: II-5-1-3 The International Civil Aviation Organization (ICAO) classifies aircraft by their indicated airspeed at runway threshold (Vat,[1]: II-5-1-3 also known as approach speed or VREF).
[1]: Table II-5-1-2 Aircraft approach categories do not change during day-to-day operation.
Approach category definitions in the United States of America are similar to those defined by ICAO.
The United States does not give a top speed for Category E.[3] Selected examples from FAA Circular 150/5300-13A - Airport Design:[4] The Northrop T-38 Talon is an example of an Approach Category E aircraft.