The value of the Coulomb (sliding) friction is non-zero only when the block is in motion. The sign function underneath the mask of this block accounts for this, as well as the change of direction. Since the dry friction force is constant, its value (Coefficient of Friction times Normal force) is multiplied by the direction of motion (from the Sign block). The coefficient of viscous friction is multiplied with the velocity to generate viscous force. Consider the following example that models the damping with just a gain block to represent viscous damping:
Instead, you could replace the Gain block representing the damping 'b' by the Coulomb and Viscous friction block to accommodate both dry friction and viscous friction. Introducing a lubricant would reduce the coefficient of static friction, but increase the coefficient of viscous friction, thus changing both input parameters in the Coulomb and Viscous friction block.
