This is related to the responsiveness of bootstrap and the “Grid System”. Bootstrap will try to automatically resize elements on your page for different screen sizes, based on how it thinks it will work best. If you want a page to be dynamic and resize on different screen sizes exactly the way you want, you can use these sizes to define how the grid will look. Here’s a helpful page with more info
tx @im-damien.
the part about Grid Classes is exactly what I needed. Grid Classes
The Bootstrap grid system has four classes:
•xs (for phones - screens less than 768px wide)
•sm (for tablets - screens equal to or greater than 768px wide)
•md (for small laptops - screens equal to or greater than 992px wide)
•lg (for laptops and desktops - screens equal to or greater than 1200px wide)
The classes above 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.