Calendar Work hours / Hours per time period

Calendar workhours/day is used to calculate the end date of the activity according to the duration of the activity.
The start date is calculated through network calculations and the end date is calculated based on the duration.

The Hours per period in admin pref is used to convert the data from one unit to another when you change the units in PM.
ie. if say your are working in hrs unit and then you change the units to days while woeking on your project, Primavera makes the calculations based on the units given in hours per period.