MOVEit DMZ can accept and store AS3 messages and AS3 MDNs that will be processed later by MOVEit Central or any other AS3 client. MOVEit DMZ, rather than MOVEit Central, is used in the role of an "AS3 server" because MOVEit DMZ already serves the function of a secure, Internet-exposed FTP(S) server.
No additional license is required to accept and store AS3 messages and AS3 MDNs on MOVEit DMZ because, according to the AS3 specification, any FTP server can function as an AS3 server. (That is, if you have licensed a MOVEit DMZ server, you already have an AS3 server.)
AS3 messages and AS3 MDNs are uploaded and downloaded through FTP and are thus part of the "normal" MOVEit DMZ file system. More specifically, all AS3 messages and AS3 MDNs will be found in the "/Home/..." or "/Distribution/..." folders and are otherwise treated as "normal" files.