The Linear System Editor works on continuous time as well as discrete time system. Pressing the s<->z button will transform a continuous-time linear system into a discrete-time linear system and back. You can choose between a Bilinear transformation (Tustin), a Forward Euler transformation and a Backward Difference transformation. When a continuous-time system is transferred, the user is asked to specify the sample time of the discrete-time system.
If you want to transfer a linear system directly (i.e. replace the z by and s or vice versa), click the Edit button and select or deselect the Discrete Sample time.