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Typically, when you compare two or more XACML requests, there are variants such as date time in the environment section. But many XACML based decision systems have one or more of the (subject,resource,action,environment) sections to be empty or invariant, thereby bringing in that constant factor to xacml requests in a system.  Example: The XACML policy in your system does not concern with any date or time or any environment factors. In this case, it may happen that for a single subject (such as an user logging in), the requests are going to be identical.  In a system, when there are going to be the same XACML requests coming to the PDP which will lead to identical responses, it makes sense to cache the decisions previously made by the PDP. This is what we call "Decision Caching".