The Manfrotto 250SP1 Retractable Spiked Foot is an optional spike/rubber foot for the "D25" 25mm aluminum tube found on the 681/681B professional monopods, made from durable rubber and anodized aluminum alloy so that it will be extremely resistant to corrosion.  Having a spiked foot is beneficial anytime you're shooting upon soft earth surfaces.  

• Retractable spiked foot made from anodized aluminum
• Fits the 25mm diameter aluminum tube of the 681/681B monopods
• Concentric, hard-wearing rubber covering easily extended for use upon hard surfaces