This article is for Drupal 7.x. For Drupal 8.x see Working with breakpoints in Drupal 8.
The Breakpoints module allows you to group breakpoints together. This way other modules can use these groups. The Picture module, for example, allows you to select a breakpoint group when you select the Picture display formatter for a field. The formatter will determine the size of the image to display based on the breakpoints in the selected group.
Following these steps you can create your own groups:
Navigate to the breakpoints configurations page
Go to Configuration » Media » Breakpoints. You will notice you can add your own breakpoints on this page.
In stead, click on "Add a new group":
Configure the breakpoints group
Enter a name for the new breakpoints group and select the breakpoints to include in this group:
Save the form and your new group should show up at the top of the page. You should also see the breakpoints selected on the previous step in this group:
Make sure your breakpoints are ordered from 'small' to 'large', because most modules depending on breakpoints will use the 'largest' one.