In the absence type configuration, you need to set up the minimum leave duration. Please go to Configuration -> Absence Types → Select your absence type → Edit
Open the Requesting tab and select the desired absence period. Values allowed here are different from days based and hours based absence types ("Duration unit" configurable while creating a new absence type). Afterwards, don't forget to click the Save button :).
3. Now, you can go to the 'Apply' tab and select a particular day. At the right side of the screen you should be able to choose the specific hours or half day absence.
Remember that this configuration is applicable only to the selected absence type. You have to repeat the above steps for each absence type with that kind of policy.