Hello Conrad
This is an excellent question, and it has given the opportunity for further research. It seems that Cisco has predefined various ports for use with TTY lines. Specifically, for Async ports, you use the absolute numbers of the Async ports (1 to 16 for example) and prepend 20 at the beginning. In the book CCIE Practical Studies Volume 1, page 55, it states:
Lines 1 through 16 are TTY lines. you use these lines for the reverse Telnet sessions. To configure a reverse Telnet session, simply add a 20 in front of the absolute line number. The syntax is in one of the following two forms. From the console mode, it is
telnetip_address20xxwhere xx is the absolute line number (in this case 1 to 16)… The next way to configure reverse Telnet is entered from configuation mode, in the form of an IP host table. From the configuration mode, use
ip hosthostname20xxip_address.The IP address used should be the one on a loopback interface. This way you can use a reverse Telnet session while other physical interfaces are down.
I have also found that Cisco has defined ports 20xx for use as TTY rotary lines. Rotary lines seems to refer to the async type connections.
Finally, I have also found that there are various port ranges that are used for different Telnet access scenarios. In the High-Speed Cisco networks book by John Vacca, on page 511 it shows the following table:
Here you can see that the 2001 to 2999 range is used for AUX and Terminal TTY ports.
So ultimately, I believe it is just a matter of convention. Use 20XX and place the absolute number of the async connection where the XX is and there you go.
I hope this has been helpful!
Laz