No. 98 and No. 99 work as a pair. One cuts left, one right — widening grooves and dadoes from the side.
Construction
Each plane is a small rectangular body with the iron set at the side, cutting horizontally into the wall of a groove or rebate. The No. 98 cuts from the left side, the No. 99 from the right. Typically sold together as a set.
Use
Widening an existing groove or dado that is slightly too tight for a panel or divider. Adjusting the wall of a rebate after the main cut. These are corrective planes — you reach for them when the fit is close but not right.