UML2.X sequence diagrams (SD) permit the modelling of behaviours of systems. With the combined fragments (CF) that can be nested and aggregated in a single SD more complex behaviours can be created. However the standard interpretation and the consideration of all the aspects related to the CF are not obvious to handle, this represses their proper use. We proposed in a previous work, a causal semantics based on partial order theory, which is suitable for the computation of all possible valid traces for UML2.X sequence diagrams with nested CF modelling behaviours of distributed systems. In this work, we deal with an important aspect that is the guard evaluation and the synchronization between the lifelines that are complex issues.