This function currently returns character CREATED order. However, it has not been updated since the ability to manually re-order characters was added to the game, rendering it largely useless (for getting character order anyway; getting the ID for each current character name is still handy).
EDIT: It should be clarified that the function itself does not actually return ANY order as one of the return values. Instead it is used in a for loop to iterate through the table it returns, which is ordered by creation date.
For example: esoui/pregameandingame/zo_addonmanager/zo_addonmanager.lua
Code:
for i=1, self:GetNumCharacters() do
local name = self:GetCharacterInfo(i)