This handy set is a stem to fit around the seat post of an eZee Expedir electric cargo bicycle(28.6mm outer diameter) and a handlebar with grips to provide a safe place for your first passenger to hang on to. The handlebar itself is a standard clamp diameter (25.4mm) so if you need to you can interchange it with different handlebars to get the shape and size to suit you.
Generally speaking this is what is used when a child grows out of their baby capsule at around 5 years old and you still want them on the back of your long-tail cargo bike.
This is usually purchased in conjunction with the pillion seat (cushion). A similar option for the second passenger to go behind this one (if you have 2 kids 5-10 years old) is the childseat for 5-10 year olds.