Some restaurants will print a label with some customer information to attach to a cup, take out bag, etc.

The Online Order application can trigger the automatic printing of a label. This label has to be an "Easy Label" provided by 7 Spaces, a 3rd party app provider in the Clover App Market. 

If you are using this app, have the printer required for this app, and have it properly setup; then you can have Online Order also print these labels. 

The Online Order application installs an "Order Alert" app on the Clover device(s). This Order Alert app is where you enable the auto label printing. 

Check out this detailed Solution how to setup the Order Alert app. Number 9 is the section that applies to the label printing.  If you have the Easy Label app setup properly, it's basically turning on one setting from the Order Alert app.