The Collaborative International Dictionary of English v.0.48 defines a mechanism as:
The series of causal relations that operate to produce an effect in any system; as, the mechanism of a chemical reaction.
This is, a mechanism is a function that produces some result. As mechanisms determine the type of result, they necessarily impose some policy (rules). Mechanisms that are referred to as policy-free are those that minimally impose policy.
A process can be used to encapsulate a program instance. On Unix, a process is associated with a UID. This UID, in part, determines what the process is authorized to do. On a microkernel, the policy that controls what resources a process may access are realized separately. This may achieved through the use of UIDs, however need not. Thus, the processes mechanism on such microkernels impose less policy than on Unix. A process is not policy free as it is possible to use other mechanisms to encapsulate a program instance, e.g., SFI and tainting ala Asbestos.