@V, Gukan All Resource Manager template functions are available to use within a policy rule, except the following functions and user-defined functions:
- copyIndex()
- dateTimeAdd()
- deployment()
- environment()
- extensionResourceId()
- listAccountSas()
- listKeys()
- listSecrets()
- list*
- managementGroup()
- newGuid()
- pickZones()
- providers()
- reference()
- resourceId()
- subscriptionResourceId()
- tenantResourceId()
- tenant()
- utcNow(format)
- variables()
For more information refer this document.