Smart-T is for use with DDB or NDB only.  It allows multiple light strings to be connected to a single output port.

It is waterproof and has 1 male and 2 female 3-pin connectors.   The 8" Smart-T is about 8" across and the 12" Smart-T is about 12" across.   Both have a 3" drop cable.

There is a hook on top for hanging in the open. 

It can also be hidden from view by mounting it above a panel, with only the center drop wire hanging down through a hole (0.725 inch diameter).  It works well with panels about 0.1 inches thick.