User Guide: helping clients, attracting prospects
Technical documentation is notoriously known as something of no use when you try to start your brand new vacuum cleaner. Though it is seldom regarded as a marketing tool, user guides are the documents clients read the most.

Sales engineers often use user guides as a sales argument, if the technical documentation takes into account pre-sales. Within the company, technical documentation then changes its status from a costly after-sales service to an efficient marketing tool.
Journalists also use users guides to find detailed information about a solution and test them alongs with the product. User guides are seldom communicated to the larger public. The technical level of a user guide depends on the technical expertise of its audience. But a good user guides is always as simple as possible.
I would like to know more about DITA. Is it commercial version only?
DITA XML is an open OASIS Standard. Commercial or non-commercial authoring and publishing tools are available.
As far as I’m concerned I use a 100% free DITA XML authoring and publishing solution based on Emacs and Dita Open Toolkit.