Global illumination provides more realistic, indirect lighting to a map. It generally looks best when used with indoor/interior maps having a low energy directional light. It's an easy way to add better quality lighting to a map, however it comes at the cost of performance so it should be used cautiously (and should also be considered experimental at this time). Global illumination can be enabled from the Map Properties panel.
The images below show an example map before and after global illumination has been enabled. As you can see in the second image (which has global illumination enabled), corners and areas that are obscured by objects appear darker and the lighting is generally more natural looking compared to the default lighting.
The “Subdivisions” setting determines the quality of the lighting. A higher number of subdivisions takes more performance, but it's best to experiment as to what looks best for a particular map. It's recommended to start with the lowest value and then work your way up if you're not satisfied with the result.
Note that the “Bake” button needs to be clicked after changing the number of subdivisions or modifying the “Use in Baked Light” setting for an entity. The lighting also needs to be baked again if the overall size of your map is changed.
In some cases, it's best to exclude certain entities from being included in the global illumination calculations. The “Use in Baked Light” setting on the Entity Properties panel can be unchecked for an entity to exclude it from these calculations so it's only affected by the normal lighting. This is enabled for entities by default, and should typically be disabled for characters or animated objects that change or move significantly. As mentioned above, the “Bake” button on the Map Properties panel should be clicked again in order for the lighting to be properly updated after modifying this setting for any entities.
In the images below, the map on the left has not had any entities excluded from the global illumination. In the map on the right, the characters and container objects have been excluded. This can also serve as a subtle indication to the player of entities that can be interacted with in some way.