margin bottom value in bootstrap code example

Example 1: bootstrap Margin and padding

Use the margin and padding spacing utilities to control how elements and 
components are spaced and sized. Bootstrap 4 includes a five-level scale for
spacing utilities, based on a 1rem value default $spacer variable. Choose
values for all viewports (e.g., .mr-3 for margin-right: 1rem), or pick
responsive variants to target specific viewports (e.g., .mr-md-3 for 
margin-right: 1rem starting at the md breakpoint).

Margin Y 0
  
Margin Y 1
  
Margin Y 2
  
Margin Y 3
  
Margin Y 4
  
Margin Y 5
  
Margin Y Auto

Example 2: spacing bootstrap

The classes are named using the format {property}{sides}-{size} for xs 
and {property}{sides}-{breakpoint}-{size} for sm, md, lg, and xl.

Where property is one of:

m - for classes that set margin
p - for classes that set padding

Where sides is one of:

t - for classes that set margin-top or padding-top
b - for classes that set margin-bottom or padding-bottom
l - for classes that set margin-left or padding-left
r - for classes that set margin-right or padding-right
x - for classes that set both *-left and *-right
y - for classes that set both *-top and *-bottom
blank - for classes that set a margin or padding on all 4 sides of the element

Where size is one of:

0 - for classes that eliminate the margin or padding by setting it to 0
1 - (by default) for classes that set the margin or padding to $spacer * .25
2 - (by default) for classes that set the margin or padding to $spacer * .5
3 - (by default) for classes that set the margin or padding to $spacer
4 - (by default) for classes that set the margin or padding to $spacer * 1.5
5 - (by default) for classes that set the margin or padding to $spacer * 3
auto - for classes that set the margin to auto

$spacer defaults to 1rem