In-Line Eductor (E63/L6) - Simply means the device is placed somewhere in the line lay.
By-Pass Eductor (E61) - This device is also an "In-Line" eductor but includes a ¼ turn valve which switches from venturi effect/foam pick up to straight water.
BOTH type have a straight water flow option.
An In-line eductor's metering valve is set to "0" or By-Pass educator can be set to water flow. These features will be important for flushing hose lines of foam solution but DOES NOT flush the eductor pick up tube and some internal parts.
Either type eductor should be flushed thoroughly with water after use for at least 3 MINUTES.

An eductor uses "Venturi Effect" to pull foam concentrate from the bucket and into the eductor's chamber. If concentrate is being educted, foam solution is created and exits the eductor at this point.

Things that effect eductor function
Inlet pressure +/-
Effects after eductor (hose length/size, nozzle used, elevation of nozzle in relation to eductor)
Nozzle open, closed or partial positions.
Viscosity of the foam concentrate
Elevation difference between eductor & concentrate
Metering valve setting (0, 1, 3, 6% etc)
Internal function - Metering valve parts may stick if not cleaned properly from last use or "gunk" left in pick up tube from last use.