Currently, the single select field supports for numeric values only. It would be helpful to being able to assign alphanumeric values.
For example, looking at the screenshot I’d like to make code values not just as a number of days but named like 30/70, 50/30/20, etc. There can be cases other than just payment terms like the ones I have here for this case.
You can add your own fields, of any of the primitive types, including text fields.
The Value field is there just to make the most common use case easy to achieve.
Hey Chris, I don’t understand what you mean. 
It’s not possible to assign string values - only the numeric values.
On entity view, if you open a single select option (using alt click) you can add whatever fields you like.
(A single select is basically like a mini database)
@myg_ge I believe Chris is referring to this:
Holy Mama! Thanks @Michael_Ichter and @Chr1sG - I knew something new today.
I thought that “mini database” thing was related to workflows only. Interesting
