The term subsumption refers to an architecture first proposed by Rodney Brooks of the MIT Mobile Robot Lab.  A subsumption architecture allows the architect of the software to determine which lower-level, self-managed behaviors should be subsumed by other higher-level layers of the architecture. 

A simple example might be a simple robot that has three behaviors.  First, lets say the robot has a simple behavior that monitors the overall battery level.  Second, there is a second behavior that just randomly navigates the robot around without hitting obstacles.  Third, there is a battery charger behavior that follows a prescribed path to the battery charger and charges up the robot. 

Now consider the robot when the battery charge is good.  When this is the case, the roaming behavior is the top level behavior and the robot just wanders around. 

What happens when the battery monitor behavior notices the battery charge getting low? The battery charger behavior subsumes the roaming behavior and the robot navigates to the charger to charge up. 

These subsumptive layers can be added on top of one another to create complex behavior modifications and reactions to outside stimulus. 

Subsumptive architectures are easy to implement on multi-tasking operating systems or parallel processor designs.  Several behaviors can simultaneously run with a arbitration process determining the current subsumption scheme for the given conditions.