SalesParmTable ParmId and TableRefId

Why both?

ParmId is initialized using NumberSeq::newGetNum(CompanyInfo::numRefParmId()).num().

TableRefId is initialized using FormLetter::getTableRef().

Both columns get their value from the same number sequence. SalesParmLine gets its values from SalesParmTable. We're burning twice the numbers for aparently no gain. Why did MS add TableRefId, which is not a table ID, to the Parm tables?