Policy assemblies are assemblies used during policy evaluation of a policy level by the security system. If you add a custom ...

Policy  assemblies are assemblies used during policy evaluation of a policy level by  the security system.  If you add a custom security object such as a  custom permission or custom membership condition to policy, be sure to add  the assembly implementing the custom object and all the assemblies it depends  on to this list.  The policy system will grant these assemblies full  trust for the policy level to which this list belongs.