A dropfile is a simple binary or text file used by a bulletin board system (BBS) to pass information about the BBS itself and the current user to an external BBS door.
Many different dropfile formats exist; most of these are application-specific. While there was never a "standard" dropfile, there were several formats that were commonly used. However, little documentation has survived for most of them. General formats are known, but specific values for certain fields or meanings for the actual fields have been lost with time.
This format originated in the GAP BBS application and quickly became a de facto standard. The original DOOR.SYS contained 52 lines. Nearly all modern BBS applications support this format.
RBBS-PC used this format exclusively. This file is 13 lines long. The name of the dropfile never changed, even if it was used in multi-node environments.
Used by RemoteAccess, this allowed slightly more flexibility for one to nine nodes. This is the exact same format as DORINFO1.DEF.
This was used by WWIV BBS and contains 32 lines.
Spitfire (BBS) created this dropfile.
This binary dropfile was created and used by PCBoard, FeatherNet PRO, and others.
An uncommon dropfile created by 2AM-BBS.
Created by Maarten Berkers, author of EleBBS, and James Coyle, author of Mystic BBS, this dropfile is able to handle  connections created by newer BBS software (EleBBS, Mystic BBS, Synchronet) and by certain older BBS applications (mostly OS/2-native: Maximus, Telegard, AdeptXBBS).
Wildcat! BBS 3.0 and above create this file for additional information not supported by other dropfiles.