The Manfrotto 204SCK3 Suction Cups with Retractable Spiked Feet (a set of 3) fit on their "D20.4" carbon fiber 20.4mm outer diameter tube found on the 055CX3 and 055CXPRO3 tripods.
The suction cups increase the 055 tripod's effective footprint by
~20x, yielding a significant increase in stability when needed for
certain surfaces. Retract the rubber cup to reveal a steel tip which
provides a firm footing on soft ground.
These tripod feet can be removed and replaced at will, even in the field.