Observer and target move with 3D velocities V_O and V_T in the cosmological frame. Their transverse component V_{O ⊥} and V_{T ⊥} determines the transverse velocity V_{M ⊥} of the impact point M. The latter combines with the transverse velocity of the absorber V_{A ⊥} to define the relative drift velocity of the LoS with respect to the gas, V_d, which sets the scale probed within the absorber during a given time interval.