Navmeshing with the Preopt
The Preopt.obj is the pre-optimized version of the navmesh that provides an accurate version of the ground. The Picture below is a Navmesh and Preopt loaded into 3dsmax.
The Navmesh is commonly optimized over large areas to the point where it not longer accurately reflects the ground area. When the difference is more than about a meter/yard, the game can crash when a bot crosses over that area. DICE only Navmeshed small combat areas, so this was not a problem when the tools were released. It only developed as an issue when the community tried to navmesh larger sized maps.
Therefore, the preopt is a very useful tool in making sure that the navmesh is accurate to the ground. The preopt.obj is in the same location as the infantry.obj and vehicle.obj navmeshes. Simply import the preopt.obj the same way as you imported the navmeshes while you already have a navmesh open, however, you do not need to add the 'materials.mtl' line, Without materials, the Preopt stays gray colored and is easier to distinguish. On import 3ds Max will also ask you for a new object name, The default name suggestion is fine.
In order to prevent modifying the preopt, select the preopt, right-click on it and click Freeze selection.
The preopt will turn dark gray and you'll be unable to select it. Select the navmesh to bring it up on top.
To delete the preopt before exporting, right-click anywhere in viewport and click Unfreeze above the Freeze selection, select the preopt and delete.