ISDN standards define certain key interfaces of the network as Reference Points.
Reference Point T is on the user's side of the network termination and allows a single TE or TA to be connected.
Reference Point S allows more than one TE or TA to be connected. It only exists for Basic Rate ISDN.
In this configuration, Reference Point S and Reference Point T are effectively in the same place. The only distinction is the number of devices attached and so this reference point is often referred to as S/T.