With regard to the grid layout in bootstrap, what is the significance of the coexistence of xs and sm?

look at a sample website layout in the material using such a piece of code

< div class= "row" >
< div class= "col-sm-4 col-xs-12" > < / div >
< div class= "col-sm-4 col-xs-12" > < / div >
< div class= "col-sm-4 col-xs-12" > < / div >
< / div >

it seems unnecessary to have three col-xs-12 here?
as long as the page size is smaller than sm, each block will automatically wrap to fill up

Grid-system bootstrap

Mar.18,2021

col-xs-12 indicates that when the screen width is less than 768px, the container width of the class is the same as the screen width
specific reference:

https://v3.bootcss.com/css/

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