//ash/content contains code that has dependencies on //content. Most code here is Chrome OS-specific WebUI for system web apps.
General purpose window manager or system UI code should not have content dependencies, and should not live in this directory. Prefer a different top-level ash directory, like //ash/system, //ash/wm, or add //ash/your_feature. Low-level components go in //ash/components/your_feature.
Each subdirectory should be its own separate “module”, and have its own BUILD.gn file. See this directory's BUILD.gn file for tips on adding your own subdirectory.
This directory is in //ash because it runs in the “ash-chrome” binary when Lacros is running. Most of its subdirectories used to live in //chromeos/components. See the Lacros documentation or the Lacros directory migration design.