Technical documentation is often considered as a synonym for how-tos and user manuals. But it is actually something different.
Technical documentation does include user manuals, integration, and installation guides. But this type of content rather consists of dry facts.
Another difference is that it may only be available for internal use. Last but not least, technical documentation may describe not only the features of a software, but it can also document workflows inside a company.
The latter kind of technical documentation may contain charts and other visual material. It explains how something is supposed to work and emphasizes why it is done in such a way and not the other.
Good technical documentation starts with a business model. Then it describes working routines in the company. It goes down to the product and ends with end-user manuals. Technical documentation shows how to use the product and how it is actually created.