Described is a data model used in a continuation based runtime that executes activities. The data model provides for declaring parameters to define data flow direction (in, out or both in and out) with respect to activities. The model further allows for the declaring of variables to provide for data storing and sharing between activities that are in scope, based on a tree relationship between activities. In one aspect, an activity tree includes a topmost parent composite activity and at least one child activity of that topmost parent composite activity. Variables are associated with one or more composite activities of the tree, and the ancestral relationships between composite activities and (non-composite) child activities determine the in-scope access to variables. Arguments such as binding expressions may bind variables to an activity's parameters.