The Core Croquet model describes a complete independent simulation, in which changing behavior is automatically synchronized between all participants. The model is best thought of as describing replicated objects with behavior over time, rather than as older models of program and data (or state). The collection of objects in a given replicated simulation is called an “island.”
However, one of the nice things about the Croquet model is that it can co-exist with other models. It is perfectly reasonable to use a “data” model for immutable objects (which do not change their behavior over time).