Long ago, Phabricator organized code primarily in terms of "Arcanist Projects". These were roughly human-readable labels for subdirectories in a repository, like "frontend-server" or "internal-tools". This was somewhat natural in large SVN repositories, where one repository often contained multiple logical projects (each with its own independent codebase and rulesets), and these were the first kind of repositories Phabricator worked with (at Facebook, circa 2007).
Subversion is much less popular now than it was in 2007, and Git and Mercurial are much more popular. With Git and Mercurial, it is far more common to organize code in many different repositories, where each repository is one logical project. In this case, the concept of "arcanist projects" isn't useful, because they always map directly to repository roots but add an unnecessary layer of indirection, complexity, and confusion. To make this problem worse, "projects" have become a much more core part of the product, and many users experience significant confusion when they learn that "arcanist projects" and "projects" are entirely separate things, even though they basically have the same name.
Phabricator's integration with repositories and tooling for segmenting rulesets for a codebase has improved over time, and we've reduced the importance of "arcanist projects". Circa 2011 they were mandatory; circa 2012-2013 they became optional; we are now removing them entirely. We believe they are virtually unused by modern installs and that installs with large SVN repositories now have other tools available which can serve the same purposes in almost all cases without requiring us to retain this confusing relic of a bygone era.