Regional densifications of the ITRF (i.e. EUREF in Europe, SIRGAS in Latin America, etc.) are necessary to provide a reliable accessibility to the global reference frame and to support the appropriate use of positioning and navigation techniques. The main objective of these regional densifications is to guarantee the consistency between terrestrial reference station positions and satellite orbits, which have to be determined in the same reference frame in order to exploit the high-precision of the measurements. Special care has to be given to the datum realization. Today, the reference frames are a set of fiducial stations with known epoch positions, which are transformed to other epochs by applying constant velocities, i.e. linear coordinate changes are considered only. Nevertheless, station positions show seasonal variations resulting mainly from a combination of geophysical loading. Ignoring these seasonal variations, and other non-lineal movements, may introduce large errors in the datum realization and the reference networks can be significantly deformed. These effects are larger in regional networks than in global ones. According to this, the main purposes of this task are to define the best strategy for the datum realization and observation combination within regional reference frames, and to improve the modelling and representation of their kinematics.