The object policy is a very minimal policy with just things like list, load, view, create, and remove. It's not used very often.
The resource policy has those permissions plus resource-specific permissions like duplicate, add children, move, publish, unpublish, etc. If you're dealing with resources, you almost always want to use the resource policy.
Of course those policies are just examples, since you might want to give some users create permission but not remove permission, for example, or give users the right to delete, but not remove resources (they get the line through them in the tree, but can't be removed from the DB). In those cases, you could duplicate the resource policy, assign the duplicate, and uncheck some of the permissions.
Hope that makes sense.
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