Use of installer's interactive shells on tty1-tty4

[Richard Owlett]

Where can I find description of using installer's interactive shells on 
tty1-tty4?

All I've found are passing mentions in
  https://www.debian.org/releases/stable/amd64/ch05s03.en.html
  https://www.debian.org/releases/stable/amd64/ch06s03.en.html