You may have a limited capacity for part of your event but still be selling different ticket types. For example let's say you have a maximum capacity of 500, and there are tickets for both adults and children. You want to be able to sell up to 500 tickets of each but make sure the total doesn't exceed 500. This is simple with ticket groups.
Navigate to the edit event form or create a new event and from there:
Create all your ticket types and set the quantity to maximum available.
Click on Add a new ticket group.
Give your group a name that represents your tickets.
Under Add tickets, tick the box for each ticket that should be limited by the maximum quantity.
In the filed Max quantity for tickets sold in this group enter the maximum quantity (e.g. for the example above this would be 500).
Click Save ticket group.
That's it! Now the tickets in that group won't sell more than the maximum quantity specified.