SPD-HR21 is a 4-Pin Push-Power (compatible with Hirose 4-Pin) to 2.1mm DC barrel jack. The SPD-HR21 is meant to be used with Hirose-compatible power distros to 2.1mm DC barrel jack devices.
This Hirose 4-Pin port-compatible cable is capable of working with the following;
Lectrosonics UCR411
Lectrosonics UCR401
Lectrosonics UCR211
Lectrosonics UCR201
While it can work with any 2.1mm DC barrel jack device, note that this cable is unregulated and will output 9~18V. Do not use with 2.1mm DC input devices that require regulated voltage.