Fig 1. Reduce/Multiply

First use <G> to mark out the area you want to modify, then you have the option to click one of the shortcut buttons along the bottom, or enter a specific target into the Num or Percentage fields. If you enter a value into either of the target fields, make sure you type <Enter> to update the other field, then click on the Do It button to perform the operation.

The following additional parameters apply to decimation only:

Feature Weight 

Set this percentage higher to retain more faces in areas of greater detail. Set it lower to more evenly decimate faces across the marked areas.

Smooth After Reduce 

After the faces are decimated, a subtle smoothing is applied to the marked areas to help get rid of thin triangles. This is recommended if you are going to reduce the marked areas to even lower levels of decimation.