Seaquel
SQL Server error code

SQL Server error 6826

SQL Server raises error 6826 at severity 16, which marks a problem with the query or data that you can correct.

Every IDREFS or NMTOKENS column in a FOR XML EXPLICIT query must appear in a separate SELECT clause, and the instances must be ordered directly after the element to which they belong.
Msg6826
Severity16
Group6000–6999

Source: Database Engine events and errors