Here is text from the page for the 2021 edition that answers your question, at least in part. "This current 2021 release is a major upgrade in that new MSS have been included, corrections have been made to numerous readings and variant divisions, and new coding has been added both to enhance the information for the user and to enable some advanced uses of the database, with some of those uses being present in the update and others on the near-term horizon for implementation. Beyond this major update of the Apparatus, an advantage of this electronic format is that additions and improvements can carried out on a continual basis, with some already being planned with the inclusion of more witnesses. The result is a product that continually improves in both quality and comprehensiveness."