What's the difference between auto and manual modes in update-alternatives?

Question

I installed vim and ran update-alternatives. Then I installed vim-full and ran it again. I noticed that there are two entries for vim.basic: one in auto mode, and one in manual.

What's the difference and what does it matter?

Answer 1

automatic mode
          When  a  link  group  is in automatic mode, the alternatives system ensures that the links in the group
          point to the highest priority alternative appropriate for the group.

manual mode
          When a link group is in manual mode, the alternatives system will not make any changes  to  the  system
          administrator's settings.

Like the above excerpt from man update-alternatives says:

1. Automatic mode:

   Changes will be made by the system to the settings of the administrator of that system if this change will ensure that the best option or choice of settings will always be used for that application(s).

2. Manual mode:

   Changes will not be made by the system to the settings selected by the system administrator. And so the system admin's choice will be kept and used at all times.