[cflsystems]

I believe these are language variables and probably hardcoded which ones to select and include in the dropdown list. You will need to find the template outputting the values and the php class responsible for it and modify it if you want to extend this.
However I find it odd that anyone will want to include this. It is very uncommon to allow something like this. No one cares to fill it out and if you modify this to allow customers to enter free text you may just end with some gibberish as value for the most part.



@Ed - it is part of the address fields setup in admin and it is off by default. I am even surprised XC5 has this included by default yet it is missing street second line and company. It makes absolutely no sense but hey it is XC after all....