What does choosing "Manual Package Selection" allow me to do, versus choosing one of the other options?
Check out the Ubuntu Server Guide, under Package Tasks:
Manually select packages: Executes
aptitude allowing you to individually select packages.
(Aptitude is a Text-User-Interface for package administration)
If anyone is looking for the updated link Ubuntu Server Guide » Installation .
This link is for all LTS releases.
On Ubuntu 16.04.04 it seems to do nothing. After choosing this option the system starts installation and then reboots without ever giving a choice for individual packages.