What is "Include in Map Extent" and how does it work?

Include in Map Extent is a feature is a little difficult to understand. The basics center around the idea that Cartographica looks at the visible area in the map as the sum of all of the "extent" of the layers. The extent is considered to be the minimum bounding rectangle around the features in a layer. For most layers, the full extent is added to the extent of every other layer in order to determine how large a map is shown in the Map Window.
However, in the case Live Maps, by default their extent (the whole world) is excluded from the calculation of the size of the map. In this way, the Map Window concentrates on the area of other layers, so as to make the Full Extent of the Map the minimum bounding rectangle of all of the layers, except the Live Map. The result is that the Map Window is focussed on the user content, not the whole world. In most cases, this is a desired effect.
There are some times when using the Full Extent of all non-live layers shows too much area. Examples would be when using tiled satellite imagery, where you have 4 tiles that come together at the center of a vector layer. If the tiles each cover a 10km x 10km area, but the vector layer is only a total of 4km x 4km, displaying the interesting (vector) data as a small section in the middle of the larger 20km x 20km map is often not desired. In this case, uncheck the "Include in Map Extent" for the raster layer(s) and the Map Window will display only the area directly surrounding the 4km x 4km vector layer.
This technique can also be employed to show sub-sections of vector data when used with smaller vector sets. For example, a citywide street map vector layer could be used by reference in a map containing data about just one neighborhood if Include in Map Extent was unchecked. In this way, that street layer can be reused in other Cartographica Map Sets and included "by reference" instead of including the data directly, allowing for easy sharing of the streets layer amongst many Map Sets.

For an example of use, take a look at How do I add more space around my map in Map Layout