Can be either - dependant on buisness requirements
16 hours tend to be two 8 hour shifts over two days.
8 hour days
16 Hour shifts are based over two days each of your shifts are 8 hours long the same would be for a contract of 24 hours you would work 3 days a week each 8 hour shifts
A 16 hour contract would be generally over 2 days, sometimes 3 with one of the 8 hours being split over 2 days.
It all depends on the needs of the business.