What is bd-highlight in Bootstrap 4.1?

On the bootstrap 4.1 documentation page for flexbox, there are several examples where the class bd-highlight is used. For example:

<div class="d-flex p-2 bd-highlight">I'm a flexbox container!</div>

What is the use of bd-highlight?
There doesn't seems to be any documentation for this class.

I think this is just a custom css class to highlight the examples, i.e., set background color and borders, on the documentation pages.

.bd-highlight {
    background-color: rgba(86,61,124,0.15);
    border: 1px solid rgba(86,61,124,0.15);
}

bd = Bootstrap Documentation?

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