Mixed: Mobile And Desktop
The Bootstrap grid system has four classes: xs (phones), sm (tablets), md (desktops), and lg (larger desktops). The classes can be combined to create more dynamic and flexible layouts.
Tip: Each class scales up, so if you wish to set the same widths for xs and sm, you only need to specify xs.
Read more
https://www.w3schools.com/bootstrap/bootstrap_grid_examples.aspx