SL100 is a general purpose, medium power NPN transistor. It is mostly used as switch in common emitter configuration. The transistor terminals require a fixed DC voltage to operate in the desired region of its characteristic curves. This is known as the biasing. For switching applications, SL100 is biased in such a way that it remains fully on if there is a signal at its base. In the absence of base signal, it gets turned off completely.
The emitter leg of SL100 is indicated by a protruding edge in the transistor case. The base is nearest to the emitter while collector lies at other extreme of the casing.