Also, I was just asked for an MDP by reference example. I just built one and pasted it down below so that we can include it as a non-normative example.
{
 Â"Request":{
   "AccessSubject":{
    Â"Id":"s1",
    Â"Attribute":[
      {
       Â"AttributeId":"com.acme.user.employeeId",
       Â"Value":"Alice"
      }
    Â]
   },
   "Resource":[
    Â{
      "Id":"r1",
      "Attribute":[
       Â{
         "AttributeId":"com.acme.object.objectType",
         "Value":"record"
       Â},
       Â{
         "AttributeId":"com.acme.record.recordId",
         "Value":"126",
         "IncludeInResult":true
       Â}
      ]
    Â},
    Â{
      "Id":"r2",
      "Attribute":[
       Â{
         "AttributeId":"com.acme.object.objectType",
         "Value":"record"
       Â},
       Â{
         "AttributeId":"com.acme.record.recordId",
         "Value":"125",
         "IncludeInResult":true
       Â}
      ]
    Â}
   ],
   "Action":[
    Â{
      "Id":"a1",
      "Attribute":[
       Â{
         "AttributeId":"com.acme.action.actionId",
         "Value":"view",
         "IncludeInResult":true
       Â}
      ]
    Â},
    Â{
      "Id":"a2",
      "Attribute":[
       Â{
         "AttributeId":"com.acme.action.actionId",
         "Value":"edit",
         "IncludeInResult":true
       Â}
      ]
    Â},
    Â{
      "Id":"a3",
      "Attribute":[
       Â{
         "AttributeId":"com.acme.action.actionId",
         "Value":"delete",
         "IncludeInResult":true
       Â}
      ]
    Â}
   ],
   "MultiRequests":{
    Â"RequestReference":[
      {
       Â"ReferenceId":[
         "s1",
         "a1",
         "r1"
       Â]
      },
      {
       Â"ReferenceId":[
         "s1",
         "a2",
         "r1"
       Â]
      }
    Â]
   }
 Â}
}
And the response
{
 Â"Response":[
   {
    Â"Decision":"Deny",
    Â"Status":{
      "StatusCode":{
       Â"Value":"urn:oasis:names:tc:xacml:1.0:status:ok",
       Â"StatusCode":{
         "Value":"urn:oasis:names:tc:xacml:1.0:status:ok"
       Â}
      }
    Â},
    Â"Category":[
      {
       Â"CategoryId":"urn:oasis:names:tc:xacml:3.0:attribute-category:resource",
       Â"Attribute":{
         "AttributeId":"com.acme.record.recordId",
         "Value":"126",
       Â}
      },
      {
       Â"CategoryId":"urn:oasis:names:tc:xacml:3.0:attribute-category:action",
       Â"Attribute":{
         "AttributeId":"com.acme.action.actionId",
         "Value":"view",
       Â}
      }
    Â]
   },
   {
    Â"Decision":"Deny",
    Â"Status":{
      "StatusCode":{
       Â"Value":"urn:oasis:names:tc:xacml:1.0:status:ok",
       Â"StatusCode":{
         "Value":"urn:oasis:names:tc:xacml:1.0:status:ok"
       Â}
      }
    Â},
    Â"Category":[
      {
       Â"CategoryId":"urn:oasis:names:tc:xacml:3.0:attribute-category:resource",
       Â"Attribute":{
         "AttributeId":"com.acme.record.recordId",
         "Value":"126",
       Â}
      },
      {
       Â"CategoryId":"urn:oasis:names:tc:xacml:3.0:attribute-category:action",
       Â"Attribute":{
         "AttributeId":"com.acme.action.actionId",
         "Value":"edit",
       Â}
      }
    Â]
   }
 Â]
}