According to this Globe article, it's not just "packages" (def. 1 from the AHD: "a wrapped or boxed object; a parcel") but any bags larger than 6" x 12" x 4" that will not be allowed on the Orange Line during the convention. The article also says that "all permitted bags will be searched on these trains."
This is certainly not the information that is in the MBTA's DNC brochure. So, either the Globe writer is interpreting this brochure differently than I am, or he has another source of information.
Okay, I just filled out a comment form on the MBTA's Web site, asking them what exactly "packages" means. Watch this space for their reply.