There are now only really two types of emergency lighting modes for when there is a power cut, here is a simple explanation of the two.
Maintained Emergency Lighting
A maintained emergency light is permanently on in both mains normal and during a power cut. So you will always have light from the luminaire. Most maintained emergency lights are switchable, this means what you think it means. It can be switched on/off from your wall light switch, this enables you to have a combined mains room light and a emergency light all in one, such as the X-XLP.
Non-maintained Emergency Lighting
A non-maintained emergency light will only illuminate when there is a power cut. This will be a dedicated emergency light and cannot be used for normal room lighting. The X-DSA is a mini non-maintained emergency light that will only come on in a mains failure.
What about Sustained (combined) Lights?
A sustained emergency light is one that has two light sources, one for everyday mains room light and one that is only used during a power cut. This was because a fluorescent lamp could easily fail and so if you had a sustained light fitting you were always sure of lighting just when you needed it. With most emergency lights moved to LED, you can rely on there being a working light during a power cut, so sustained lights are slowly being phased out.