The goodness of the UE simulation provided by MC event generators and corresponding tunes can be tested by comparing predictions with available data. These **data** are generally measurements of the number of charged particles and their transverse momentum sum in different regions of the phase space relative to the direction of the hardest objects in the event. In particular, the hard object, which might be a jet, a charged particle or a Z boson, identifies a direction in the transverse plane. The transverse plane is then divided into four regions, according to their azimuthal angle: a "toward" and an "away" region sensitive to the hard scattering and its recoiling object, and two "transverse" regions, more sensitive to UE contributions. In recent measurements, the two transverse regions are further divided into separate measurements. The transverse region with the highest activity is called "transMAX" while the one with the smallest activity is labelled as "transMIN". The charged-particle multiplicity and the transverse momentum sum of the charged particles, measured as a function of the transverse momentum of the leading charged particle, are referred to as "UE observables" in the following.