In general, optocouplers are configured in a closed pairing, i.e., with the light emitter and light sensor facing each other in an opaque package.
Some optocouplers are in a "fork-interrupter" configuration, where the optocoupler is slotted between the transmitter and the sensor and can affect their signal. These optocouplers are suitable for object detection, vibration detection, and bouncing-free switching.
Some optocouplers are in the configuration of "reflection pairs", where one light source emits light and the other receives the source, but receives the light reflected by other objects. Reflective optocouplers are used in tachometers, motion detectors and reflection monitors.
The latter two are also known as "optosensors".