Holding Runs are for internal use. They can be used to create temporary space on your calendar to move bookings around or if you want to have emergency spaces for sick pets or loyal customers.
If you want to set up holding runs, you can do this at:
Account Settings > Runs > Customise Your Runs > Holding Run.
They will appear in grey on your calendar and you can drag and drop bookings in to them.
Please note: Holding Runs cannot be booked from your booking page or online booker, and they don't count towards your run count or occupancy rates.