A System sequence diagram visualises a use case, while a sequence diagram visualises a method of a class.
The elements participating (exchanging messages) in a system sequence diagram are Actors and Systems. The messages exchanged by these elements could be any type depending on the systems (from web service calls to data input from a human).
System Sequence Diagrams are used to define the input and outputs and sequence of interaction between the user and the system for a use case. They are used in conjunction with activity diagrams. in Sequence Diagrams,theres information flow in and out of a system are called messages. The users are identified and detailed messages are described.
The elements participating in a sequence diagram are objects (instances of various classes). The messages exchanged by these elements are method invocations.
First, we need to analyse and document the use cases. Each use case describes the main business task required. The system use case diagram is part of this activity.
Later we come to design each subsystem, namely each element participating in the system use case diagram. We draw one or more class diagrams depicting the classes participating in the solution. Then we can draw a sequence diagram for the most important (or difficult to understand) methods