Systems are divided according to the order (priority) of the linear differential equations that they represent in systems of the 1st, 2nd, ... nth order. In doing so, the coefficients of higher-order elements are often so small that they can be ignored. Therefore, in most cases the systems will be 1st, 2nd or 3rd order.
- A 1st order system has only one energy store (e.g. mass to be accelerated). They can only be used in hydraulic systems if the compressibility of the oil can be ignored (e.g. under favourable circumstances in the case of copy or following control valves).
- A 2nd order system has two energy stores, consisting of a mass and a spring (e.g. compressible oil) and is therefore capable of oscillating. Most hydraulic systems are 2nd order.
- In 3rd order systems and higher, the third and higher derivation of the differential equation must also be considered.