The normal map option can be accessed when you are building texture for a 3D model. The main advantage of this feature is the possibility to have a lower polygonal textured model visually similar to a high-poly model, for example, to upload a model to Sketchfab resource that represents the highly detailed model, but using a considerably smaller polygon count, thus reducing the upload time and visual response time upon navigating the model in a browser. 

The workflow for using the Normal map feature

After building the model with a certain detail level, required to your needs, make its decimated copy so that it weights less and can be loaded into the Sketchfab, for example.

1. Select Tools menu > Model > Decimate Model... option and specify the target polygon count for the low-poly model.

If you don't enable Replace source model option, thus there will be two models in the chunk.

2. To switch between the models, double-click on their label in Workspace pane or use Set as Default option in the Workspace pane context menu.

The decimated model is more "coarse", but also is more lightweight due to the smaller number of polygons:

3. After you have got two models in the chunk, make the rough (decimated) model active and open Workflow menu > Build Texture... dialog.

It is important to specify a high-poly model in the source data field for the Normal map generation. 

To view the built normal map, select Normal map on the toolbar for the Model view option:

4. After that you can transfer the model's texture from a high-poly model to a low-poly. Use the high-poly 3D Model in the Source data field, when generating the Diffuse map (Workflow > Build Texture...). 

It's important to use Keep UV mapping mode.

Alternatively, generate the Diffuse map from Images and select Keep UV mapping mode:

After the texturing process is completed, you can upload the model to Sketchfab and see that the model has sufficient visual details.