A drop file is a data file generated by the BBS before executing an external program. The drop file contains information about the user and his/her connection, such as the serial port number and security level. The door program will read the drop file and act accordingly (e.g. sending output to the correct serial port). In some cases the door will modify the drop file as a means of passing changes back to the BBS. This mechanism allows a door to be used with any BBS software as long as the drop file is generated correctly.
There are a variety of different drop file formats. The DOOR.SYS format had widespread support among BBS systems and door games under MS-DOS systems. Modern systems prefer DOOR32.SYS because the format is simpler and provides information about Telnet connections (something not anticipated when DOOR.SYS was invented).