Normal map option can be accessed when you are building texture for 3D model. The main advantage of this feature is the possibility to have a lower polygonal textured model visually similar to 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 browser. 

The workflow for using the Normal map feature is the following:

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

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



In the next confirmation dialog reject to replace the default model, thus there will be two models in the chunk:


2. To switch between the models, double-click on their label in the 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 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 source data field for the Normal map generation. 

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

 
4. After that you can transfer the model's texture from high-poly model to low-poly. Use the high-poly 3D Model in the Source data field, when generating the Diffuse map. It's important to use Keep UV mapping mode:


Alternatively, generate the Diffuse map from the 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.