An expansion region is a strictly nested region of an activity with explicit input and outputs (modeled as ExpansionNodes). Each input is a collection of values. If there are multiple input pins, each of them must hold the same kind of collection, although the types of the elements in the different collections may vary. The expansion region is executed once for each element (or position) in the input collection. If an expansion region has outputs, they must be collections of the same kind and must contain elements of the same type as the corresponding inputs. The number of output collections at runtime can differ from the number of input collections. On each execution of the region, an output value from the region is inserted into an output collection at the same position as the input elements. If the region execution ends with no output, then nothing is added to the output collection. If all the executions provide an output to the collection, then the output collections will have the same number of elements as the input collections. The inputs and outputs to an expansion region are modeled as ExpansionNodes. From “outside” of the region, the values on these nodes appear as collections. From “inside” the region the values appear as elements of the collections. Object flow edges connect pins outside the region to input and output expansion nodes as collections. Object flow edges connect pins inside the region to input and output expansion nodes as individual elements. From the inside of the region, these nodes are visible as individual values. If an expansion node has a name, it is the name of the individual element within the region. Any object flow edges that cross the boundary of the region, without passing through expansion nodes, provide values that are fixed within the different executions of the region.