The pinchweld is the multilayer seam that runs along the rocker panel underneath the car. One side is the rocker flange, one side is the floor flange. To drill out all those pinch welds is time consuming, so if the floor is good in that area and only the footwell areas are bad, then it's much less expensive to replace only the footwell areas. The photo below shows the pinch weld.

Be aware that if the rust extends up behind the pedals, then a second panel (toe board or toe pan) is usually needed to complete the repair. The half floor pan repair section just makes the turn up at the front of the pan, then stops. And it stops at the base of the rear seat step-up.