Why can I use bootstrap col classes without the number of cols and how is this interpreted? E.g. in the snippet
<div class="row row-header">
<div class="col-12 col-sm-8">
</div>
<div class="col col-sm align-self-center">
</div>
</div>
the second inner div uses col classes that corresponding dimensionally to the col classes of the first inner div. What does it mean?