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This paper aims at the clarification of the execution of IEC 61499 function block networks. A new event-discrete execution model called "handler-based execution model" (HB-XM) has been introduced for CNet in previous work that enables full automatic code generation for Petri-net based component descriptions . The HB-XM abstracts from the CNet language elements and specifies how each element is treated for implementation. A new graphical notation called XNet representation -execution nets -has been defined to represent the resulting behavioral models . The execution of IEC 61499 function blocks is defined using plain English text definitions. The model of the actual standard differs from the earlier draft. Several publications revealed that the standard still does not sufficiently and unambiguously define the execution behavior of function block networks. This paper uses XNet to visualize the different execution models and shows how the understanding of the XNet models will give natural answers to various ambiguities concerning the execution of function blocks.