"Optional dock" refers to a docking mechanism or device that is not necessary or required, but can be added or used if desired or needed. Full definition
The rest of the connectors you see on the bottom are designed to be used with optional docking accessories. (liliputing.com)
It works with RIM's optional dock accessory, but you only get a standard micro USB cable and charger in the box. (pcmag.com)
The device also plugs into optional docks for running Motorola's webtop software, enabling users to surf the full web on a bigger screen. (laptopmag.com)