Calendar: "Reserved" entry in the office calendar (Standard & Pro)

Let other users know that you are reserved in the office calendar

If you are to create a another calendar in addition to the office calendar, you can mark yourself busy in the office calendar as follows

1. Choose the right calendar from the Calendar dropdown. 
2. Activate the tab Create "reserved" entry in the public calendar

The other users can now see you are reserved during the marked time.