for cubic environment maps, the 6 textures are combined into a single 'cubic' texture map which is then addressed using 3D texture coordinates. This is required because you don't know what face of the box you're going to need to address when you render an object, and typically you need to reflect more than one face on the one object, so all 6 textures are needed to be 'active' at once. Cubic environment maps are enabled by calling this method with the forUVW parameter set to true, and then calling setEnvironmentMap(true).
Note that not all cards support cubic environment mapping.
for skyboxes, the 6 textures are kept separate and used independently for each face of the skybox. This is done because not all cards support 3D cubic maps and skyboxes do not need to use 3D texture coordinates so it is simpler to render each face of the box with 2D coordinates, changing texture between faces.
Skyboxes are created by calling SceneManager::setSkyBox.