hours per day
"The constant value that Microsoft Project uses when calculating duration and work. Taking the default of 8 hours, a one-day task will be calculated as 8 hours and an assignment of 1 unit will calculate as 8 hours also. Changing the hours per day to 6 would retain the assigned work as 8 hours and change the duration of the assignment to be 1.33 days. If the shift pattern for that day is unaltered (from the default 8 hours), the assignment (and task) will be complete at the end of that day and not within the next day. '
SOURCE; microsoft project glossary-projectlearning.net"