Not a SS#, because they arent a citizen. Allows employer to hire and apply taxes to. As to the Tax laws I have no idea.
Mexico's consulates are instructing illegals to get tax ID numbers in order to open bank accounts, but the number doesn't give the individual work authorization. Legal Permanent Residents have social security numbers. I wonder if these "tax ID legals" are in another category or if they are illegals who have applied for and received the tax ID # from the SSA.