Usability is a primary component of user experience. Here’s how Wikipedia defines usability:
Usability is the ease of use and learnability of a human-made object. The object of use can be a software application, website, book, tool, machine, process, or anything a human interacts with.
It’s all about how the user interacts with the software; in this case, it’s the online repair manual. Does it have the appropriate buttons or links where the user can easily see them? Are the menus clear and easy to navigate? Does the layout of the online repair manual work well? These are some factors that affect usability.
Of course, unless there are screenshots of a particular online repair manual, you likely won’t be able to know whether the one you get is user-friendly or not. I guess that’s where reviews come in. Usually, though, screenshots are included on most product pages. You can also try searching Google Images and see if there are any screenshots for the online car repair manual you want to research about.
Personally, I value accuracy of the content more than good user interface design. After all, you would certainly much rather have accurate information than a pretty manual. Still, having a good UI is really helpful, and it can certainly make the life of DIY mechanics much easier.