"Optional accessories" refers to items or add-ons that are not necessary or required, but can be chosen or added if desired for additional functionality or convenience. Full definition
Unfortunately, extended batteries for most phones are big and bulky, so manufacturers usually leave them as optional accessories, so consumers have to fork out extra cash for them. (ubergizmo.com)
For capacity limits when combined with optional accessories, see guidelines in respective user manuals. (austlen.com)
There are a ton of optional accessories on offer including the choice of wheels, upholstery and exterior / interior equipment. (motorbeam.com)