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First you need to understand what is RBAC. Provisioning policy in roles are used in RBAC. If you want to assign a role to an identity based on certain assignment rule or criteria then it is called RBAC.

Roles are generally bunch of entitlements or groups from various applications.

For ex: if you want AD account to be created with xyz entitlements to be provisoned using RBAC, You need to create provisioning policy for that role.
This provisioning policy will ensure to provision different attributes for AD while assigning role.