If some resources don't work full-time for the company, the best way is to enter the agreed part-time schedule in the User edit panel. To do so, go to the User section, select the user you wish to set as part time and choose the tab "Schedule"
In the above example, this user is working regular hours on Monday, Tuesday and Thursday; on Wednesday s/he is only available half day and not at all on Friday. The planner board will correspondingly show this number of hours on the specific days.
Alternatively, this setting can be performed by the concerned user in his/her own User's profile. To do so, click on the User's name / icon (top right corner) and then select "My Profile".
The daily parameters can then be entered in the "Schedule" section:
Last, especially for resources having irregular working times, you may simply book their unavailability by booking these slots as time-off.
Let's say you have a resource that's not available certain Tuesdays: simply create a project "Not available" (non-billable!) and book the appropriate days as per this resource schedule. If applicable, you can also make use of recurring bookings (say "every month").