The 'Domestic Data' option allows you to decide if you want allow your phone to use data while roaming.
There is a master setting, 'Roaming', for deciding if you want to use Sprint only networks, or if you want to allow your phone to roam onto other carriers networks when you are not in range of a Sprint tower (i.e. Verizon -- Sprint and Verizon have a business agreement to let eachother take advantage of each-others towers on a limited basis).
Right below 'Roaming' there are 2 more options to give you the further choice if you want to allow your phone to accept calls while you are roaming. 'Domestic Voice'.
And if you want to allow your phone to use data while roaming. 'Domestic Data'
I believe If you have an unlimited data plan you can roam for free and take advantage of both voice and data while roaming without charge, up to a point. HOWEVER, if you roam a majority of the time and accept alot of calls or use a LARGE amounts of data while roaming Sprint has the right to apply extra charges and even terminate your account.
(anyone feel free to correct this info as I'm not sure on the specifics)
If you are interested in the official jargon see this link:
https://manage.sprintpcs.com/output/en_US/manage/MyPhoneandPlan/ChangePlans/popLegalTermsPrivacy.htm
(press Ctrl. + F on your keyboard and search domestic. Should be the second result.)
(sounds like EarlyMon is referring to the new feature that allows you to natively track data, voice, and SMS usage. Settings -> More -> Usage)