LTE every interface consist of procedures which is a complete task consisting of message flows.
Some terms related to it.
Request Message - Initiating Message
Response Message - Successful outcome or Unsuccessful outcome message
-> So in LTE some procedure consist of both request and response message known as CLASS 1 and some procedures only request messages known as CLASS 2.
-> Only request messages are considering as always successful operation as request only reaches to entity.
E.g - In S1 Interface -
E-RAB set up procedure consist of E-RABSetupRequest and E-RABSetupResponse so its CLASS 1
Paging Procdure consist of only consist of paging message so its CLASS 2.
For more detail u can visit http://www.ltewirelesstech.com/2014/04/basic-terms-and-rules-in-lte.html