Programming with Sequences 
Programming with sequences is a prerequisite for reliable message exchange using Web Services Reliable Messaging..
It can sometimes be very important that service requests arrive at the provider side in the correct order. This is the case when database entries that depend on a previous write process are being written. If a database entry is changed that has not yet even been created, errors will occur.
To ensure that calls are processed in the correct order, calls that belong together are given a sequence and then evaluated by the provider.
Between different sequences or between Web services with WSRM and Web services without WSRM, there is no influence on the sequence –that is, you cannot define which sequence is to apply.
If proxy object methods are called as part of a sequence, these calls are processed "exactly once in order" (EOIO). If the proxy object methods are called without sequence, the methods are called "exactly once" without a particular order.
Scenarios for programming with sequences:
Exactly Once in Order in an LUW
Exactly Once in Order in Multiple LUWs
Using Sequences in Other Modules