Default partition mounts [ "Installation Guide" lacks index ]

[Richard Owlett <rowlett@access.net>]

I'm over 80 and doing first "from scratch" install since Squeeze ;}
Hardware is Lenovo R61 ThinkPad (64 bit).
I multi boot [Grub will have at least three options]:
  1. minimalist installation - primarily command line usage
  2. 64 bit Debian with maximum features
  3. 32 bit Debian - couple of applications require a 32 bit OS
  4. other installs with strong project dependencies

Today's question
At boot time, what determines which physical partition gets mounted as a 
specific directory ( /, /home, swap, and so forth )?

Please reference documentation as reading it will remind me of how and 
why I chose specific options.

TIA

P.S. - re-reading https://www.debian.org/releases/stable/amd64/ ;}!