The model is lowlevel enough so that we can express communication, allocation and garbage collection, but otherwise hides many of the lower-level details of an actual implementation. The models are rewriting systems whose terms are programs.
Evaluation is expressed as conditional rewriting and includes store operations.
Formal Models of Distributed Memory Management. Technical report, Computer Science Department of New York University, 1996. An exercise in cooperation - Dijkstra, Lamport et al.