Both the previous UDZ and the present UDZS family of zener diodes are in the SOD-323 (UMD2) package. The dimensions, physical shape, and markings are the same. The difference is in the internal construction.

The UDZ series employed a wire-bond construction, whereas the UDZS series uses a sandwich construction. For the UDZS series there is no wire-bond,  instead the die is sandwiched between the two leads.

Hence, the UDZS has the following positive attributes:
(A) Higher Surge Current capability.
(B) Higher ESD Capability.
(C) Higher Reliability. (Fewer parts are used in construction, in addition to the previously mentioned attributes contributing to higher reliability).