Similarity:
This 2 function helps know the Client IP address at the backend server.

Difference:
1, CIP just insert a header in the http GET request and inform to backend with the client IP, while USIP will replace the source IP from the SNIP with the client IP in back end connection.
2, Use SNIP will affect the backend reuse pool while CIP will not.
3, USIP can't be used with Surge protection function while CIP is not related with this and no affect.