Before proceeding to define the mechanisms provided in IEC 61499 to define function block types in some detail, let us recall the role of function block types and instances. A function block type definition describes the external interface and internal behaviour of a particular type of function block. However, a function block instance is, in effect, a working copy of the function block created using the function block type definition. In a large system it is likely that function block type definitions will be held in various libraries, for such purposes as, control algorithms , alarm management , input sensors and so on. The configuration of a large function block based system will require the selection of appropriate function block type libraries and then the declaration of function block instances based on selected function block types. Clear, precise and unambiguous definitions of function block types are therefore germane to using IEC 61499 effectively.