type:
constant
name:
EMPTY_VALUE_PREFIX
purpose:
Set a dropdown prefix which indicates no value
allowed:
A short string
default:
--
category:
segmentation

Using this, you can organise your attribute values to have some useful “empty” values.

For example for your subscribers to set their country, you can put 

-- Select your country

as the first country, which then returns an empty value when chosen. If you then make country required, choosing this value will return an error on the subscribe page.