The activation object is created when the flow of control first enters an execution context for declared function code, anonymous code or implementation supplied code.

As the activation object is created, it is associated with the execution context. On initialization, it has a property called arguments that cannot be deleted and which refers to an arguments object.

This activation object is then used as the variable object for instantiating all the argument variables. The activation object is discarded when the function returns its result to the caller. The activation object is an internal mechanism and so cannot be passed to the outside world, although members of the activation object may well be accessible to a running script.

Cross-references:

ECMA 262 edition 2 - section - 10.1.6

ECMA 262 edition 3 - section - 10.1.6