em is relative to font size of parent. HOWEVER, it is relative to its own element if it is applied to anything else other than font size. Below is example

section {
font-size: 18px
}
.abc {
font-size: 1.5em;
}
section {
font-size: 18px
}
.big-abc {
  font-size: 10px;
}
.abc {
padding: 1em; //here paddingwill be relative to .big-abc and not section tag
}
<section>
<h1 class = "abc big-abc"></h1>
</section>