These elements are used, when feasible, to alter the default fonts used to render variables and constants. Numbers, operators, delimiters and other symbols are unaffected. B renders its contents in bold, while T renders its contents in an upright font rather than an italic font. These can be nested to combine the effects, for a bold upright font. The BT element allows you to write <BT>term</BT> rather from having to write: <B><T>term</T></B>.
By using CLASS to describe the term as a vector, tensor or matrix etc. user agents can do a better job when rendering to non-visual media. Further conventions for term class names are not part of this specification.
Note: Don't use B for chemical formulae. These should be handled by subclassing the MATH element with class=chem, for instance: <MATH CLASS=CHEM>