Capsule light, as the name implies, is a light source shaped as a capsule. Unlike spot and point light sources that have a point origin, the capsule light source has a line at its origin and it is emitting light evenly in all directions. The following screenshot shows a red capsule light source:
Capsule lights can be used to represent fluorescent tubes or a lightsaber.
Capsules can be thought of as a sphere split into two halves, which are then extruded by the length of the capsule light's line length. The following figure shows the line start point A and end points B and R are the light's range: