What are normals?
A normal defines the orientation of a face or vertex, in short words it allows to know which side is considered the "outside".
It is perpendicular to the mesh surface, in the center of the vertex/face it is associated with.
To sum up simply, the normals affect the way the 3D object will be displayed in reaction to light, to determine which part of the 3D object will be bright, and which part will be dark.
Note that normal maps are indeed using normals, by detailing and adjusting normals information.
How to recognize inverted or broken normals?
As normals have an impact on the shading of the related surface, if these are wrong it will affect the appearance of the model.
A model with correctly oriented normals should show brightly the color/texture of the assigned material, with a shading that reacts logically to the lighting.
In 3ds Max viewport, the inverted (inward) normals are clearly appearing (in standard viewer display mode and default shading), as the model will be black with a bottom lining of the material's color (except for double sided materials).
The broken normals will display a weird shader behavior, very contrasted and which reacts strangely to the viewport's camera moves.
Normals behaving in 3ds Max (Standard viewer display mode)
In Augment, the inverted normals will not appear black, but to make sure the light behaves fully correctly on the model it is recommended to check that the normals point outward.
The broken normals will appear darker and with an inversion of lighting, darker on top, light from below.
Normals behavior in Augment
Also, if the face culling option (Editor > Rendering) is activated, an object with inward normals will display weirdly, as the outward of the mesh will be see-through. The mesh and shaders will only show on the inside of the object.
Normals behavior in Augment with face culling activated
Select the inverted part(s), go in the 'Modify' tab on the right, search and apply the 'Normal' modifier.
You may directly see the difference in the viewport (in standard viewer display mode and default shading). If the inverted normals are not fixed by this action, finish this procedure and follow the steps for broken normals.
Collapse the modifier before exporting your model (to do so, right click in the applied modifier list and select 'Collapse All').
You can also invert normals by selecting all faces in the related object and click on 'Flip' in the 'Edit Polygon' sub-menu.
If the normals are broken, you can first try to add the 'Normal' modifier (see "Inverted normals" part above), this may be enough to fix them.
If this does not solve the problem (particularly for older versions of the software), select the broken part(s), go in the 'Utilities' tab on the right, click on Reset XForm > Reset Selected.
Then go in the 'Modify' tab, and collapse the Reset XForm (to do so, right click in the applied modifier list and select 'Collapse All').
To finish, apply a 'Normal' modifier (see Inverted normals paragraph). The normals should now be correctly reset. These may reset inverted though, so don't hesitate to flip them back with the mentioned tools.
For more 3ds Max tutorials for Augment, check here.