The Extension Arm serves as a bridge with the male and end of an arm on either end. This will allow you to extend arms out farther than what they allow. 

Extension arms can be daisy chained together an infinite number of times, but after an extension arm is used, it must connect to either another extension arm, the upper piece of a drop arm or an attachment.

If you really wanted to, you could connect it to a spindle base, however it is completely unnecessary.

They can also connect directly to a Onefinity Base, and Drop Arm. 

It cannot connect to stepper bases (90 degrees or straight) and requires an arm to make that connection. 

The current offerings of 1" and 2" Extension Arms. 

An example of a Onefinity base, 1" extension, 2" extension, and an attachment

An example of a stepper base, Arm, Drop Arm, 1" Extension arm, 2" Extension Arm, Attachment.