Diffrence between CROSS APPLY and OUTER APPLY of SQL XML

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Sample XML :
DECLARE @XML XML = 
'<Root>
    <UserInfo Id="1" Name="Name 1">
        <CityInfo Id="2" City="City 1" />
        <CityInfo Id="3" City="City 2" >
            <PrefInfo Id="33" Name="Show Whether" />
        </CityInfo>
    </UserInfo>
    <UserInfo Id="2" Name="Name 2">
        <CityInfo Id="4" City="City 3" />
        <CityInfo Id="5" City="City 4" >
            <PrefInfo Id="33" Name="Show Temprature" />
        </CityInfo>
    </UserInfo>
</Root>'

Cross Apply :  Cross apply is basically use inner join of two xml collection
SELECT
    UserInfo.value('@Id', 'BIGINT') as UserId
    , UserInfo.value('@Name', 'VARCHAR(20)') as UserName
    , CityInfo.value('@Id', 'BIGINT') as CityId
    , CityInfo.value('@City', 'VARCHAR(20)') as CityName
    , PrefInfo.value('@Id', 'BIGINT') as PrefId
    , PrefInfo.value('@Name', 'VARCHAR(20)') as PrefName
FROM @xml.nodes('/Root/UserInfo')e(UserInfo)
    CROSS APPLY UserInfo.nodes('CityInfo')b(CityInfo)
    CROSS APPLY CityInfo.nodes('PrefInfo')c(PrefInfo)

Output :

UserId   UserName  CityId  CityName  PrefId   PrefName
-------- --------- ------- --------- -------- ----------------
1        Name 1    3       City 2    33       Show Whether
2        Name 2    5       City 4    33       Show Temprature 

Outer Apply :  Outer apply is basically use left outer join of two xml collection
SELECT
    UserInfo.value('@Id', 'BIGINT') as UserId
    , UserInfo.value('@Name', 'VARCHAR(20)') as UserName
    , CityInfo.value('@Id', 'BIGINT') as CityId
    , CityInfo.value('@City', 'VARCHAR(20)') as CityName
    , PrefInfo.value('@Id', 'BIGINT') as PrefId
    , PrefInfo.value('@Name', 'VARCHAR(20)') as PrefName
FROM @xml.nodes('/Root/UserInfo')e(UserInfo)
    CROSS APPLY UserInfo.nodes('CityInfo')b(CityInfo)
    OUTER APPLY CityInfo.nodes('PrefInfo')c(PrefInfo)

Output :
UserId  UserName  CityId  CityName  PrefId  PrefName
------- --------- ------- --------- ------- ----------------
1       Name 1    2       City 1    NULL    NULL
1       Name 1    3       City 2    33      Show Whether
2       Name 2    4       City 3    NULL    NULL
2       Name 2    5       City 4    33      Show Temprature