What does the "B" mean in the linked answer, when the query still works however I name it?

Question

Source: https://stackoverflow.com/a/56825537/3037607

Select A.[ID]
      ,C.*
 From  YourTable A
 Cross Apply ( values (cast((Select A.* for XML RAW) as xml))) B(XMLData) -- This B here
 Cross Apply (
                Select Item  = xAttr.value('local-name(.)', 'varchar(100)')
                      ,Value = xAttr.value('.','varchar(max)')
                 From  XMLData.nodes('//@*') xNode(xAttr)
                 Where xAttr.value('local-name(.)','varchar(100)') not in ('Id','Other-Columns','To-Exclude')
             ) C
 Join Source D on A.ID=D.ID and C.Item=D.TargetField

I tried this on my own database, but I can't wrap my head around what the B means in B(XMLData). When I remove the B, I get the following error:

No column name was specified for column 1 of 'XMLData'.

But when I replace it by (for example) helloworld(XMLData), the query still works...

I haven't been able to find a answer online which explains this for me.