Surer just got the terminology a bit off...there is "full sell" and "partial sell" when working a day off..however there is no "partial day off"...a day off includes two local nights..
"full sell" is around 150bd for a capt..which knocks a day off of your leave balance
"partial sell" is 50bd for a capt and does not affect your leave balance
in all cases the day off is returned to you within the next 3 roster periods, as GF only rosters the minimum 8 off in a 28 day period