A protocol configuration tells the BBS how to load and execute the code that implements a file transfer protocol. Every installed protocol must be configured, including the protocols included with the BBS engine.
The key is a short name that you can use to refer to the protocol elsewhere on the system (e.g. in menu items, configuration files, etc). The keyword is not intended to be visible to end-users; it is an administrative name that you assign to the protocol configuration.
Note that you can install the same file transfer protocol multiple times -- as long as each configuration has a separate key. For example, you are allowed to configure two variations of the XMODEM protocol (e.g. one with standard options and one with debug options). It is OK if both configurations point to the same library or executable. The only restriction is that each protocol configuration must be given a different key.
This setting tells the BBS how to launch the file transfer program.
A Plug-In indicates a .NET assembly containing code written specifically for the Thought File Transfer Protocols library. This is the best type of protocol to install because the BBS engine will have a great deal of control over the transfer.
A Script style-protocol is simply a standard BBS script. This is useful for pseudo-protocols like email attachments, faxes, etc., that are not actually serial-oriented binary protocols.
An External protocol is implemented by an outside executable. The engine will simply execute the program with the command-line parameters you designate. It will not have much control over the file transfer.