libinput provides a number of different out-of-the-box configurations, based on capabilities. For example: middle mouse button emulation is enabled by default if a device only has left and right buttons. On devices with a physical middle button it is available but disabled by default. Likewise, whether tapping is enabled and/or available depends on hardware capabilities. But some requirements cannot be gathered purely by looking at the hardware capabilities.
libinput uses a couple of udev properties, assigned through udev's hwdb, to detect device types. We use the same mechanism to provide us with specific tags to adjust libinput-internal behaviour. The udev properties named LIBINPUT_MODEL_.... tag devices based on a set of udev rules combined with hwdb matches. For example, we tag Chromebooks with LIBINPUT_MODEL_CHROMEBOOK.
Inside libinput, we parse those tags and use them for model-specific configuration. At the time of writing this, we use the chromebook tag to automatically enable clickfinger behaviour on those touchpads (which matches the google defaults on chromebooks). We tag the Lenovo X230 touchpad to give it it's own acceleration method. This touchpad is buggy and the data it sends has a very bad resolution.
In the future these tags will likely expand and encompass more devices that need customised tweaks. But the goal is always that libinput works well out of the box, even if the hardware is quirky. Introducing these tags instead of a sleigh of configuration options has short-term drawbacks: it increases the workload on us maintainers and it may require software updates to get a device to work exactly the way it should. The long-term benefits are maintainability and testability though, as well as us being more aware of what hardware is out there and how it needs to be fixed. Plus the relief of not having to deal with configuration snippets that are years out of date, do all the wrong things but still spread across forums like an STD.
Note: the tags are considered private API and may change at any time, depending what we want or need to do with them. Do not use them for home-made configuration.