This directory contains constants used by Chrome OS. It is a very low-level component and should only depend on //base. It should not contain any logic, except for simple helper functions like IsFooEnabled(). For example, while it can contain pref names, it should not do pref registration, because pref registration requires a dependency on //components.
It lives in //ash because these constants are used by Chrome OS system UI as well as the current/legacy built-in chrome browser. This is “ash-chrome” in the terminology of the Lacros project.
Code in this directory used to live in //chromeos/constants. That directory is being re-purposed for constants shared between the lacros-chrome browser and the ash-chrome system UI binary. Both those binaries run on Chrome OS.