What XML is
XML marks up data with nested named tags and optional attributes. It is verbose, strict about being well-formed, and can carry a schema that says what shape a document must have.
Enterprise integrations, government data feeds, RSS, SOAP services and older document formats.
What BBCode is
BBCode uses square-bracket tags. Table support is not universal, but table, tr, th and td are widely implemented.
phpBB, vBulletin and forum software descended from them.
What changes when you convert XML to BBCode
The first repeating element is treated as the rows. Attributes are read alongside child elements and prefixed so they do not collide with a same-named tag. Nesting one level deep becomes dotted column names; deeper structures are kept as text.
Check that your forum has table tags enabled. Many boards ship with them switched off.
What carries over from XML to BBCode
XML records 4 things about a table that BBCode has no way to hold.
An empty cell and a missing one are the same thing in BBCode, so a null and an empty string in the XML both come out blank and can no longer be told apart.
XML can hold a value that is itself a list or an object. BBCode has only flat cells, so nested values are flattened into one cell rather than being spread across columns.
A line break inside a cell ends the row in BBCode, so line breaks are replaced rather than carried through.
The keys stop being repeated: BBCode writes the column names once at the top, so the same table takes noticeably less space.
BBCode is the end of the line for this data. Reading the table back out of it is a separate and much harder problem than writing it, so keep the XML if you will need the values again.
The identifier 007 comes out of the BBCode as 007. Reading it as a number would have made it 7, and an id that changes value is worse than one that stays text.
The role "Analyst, data" survives with its comma, in one cell rather than split across two. That is the first thing to check in any converted table, and the usual place a BBCode file goes wrong.
The quotes around Jonah "Jo" Pryce are left as a plain quote character in the BBCode.