In versions of Alfresco Share previous to 4.0, the client-side JavaScript requested JSON data from the repository directly by using the proxy servlet. From 4.0 onwards, there is a new data web script (at /components/documentlibrary/data/) that requests data from the repository and processes the response based on a configurable set of evaluators before finally returning JSON data to the browser.
All configuration for, and evaluation of, Document Library status indicators, metadata templates, and actions is on the web tier instead of split between the repository and the browser.