What does the font size documentclass option actually do?

Question

When using e.g. the [12pt] option for a document class like

\documentclass[12pt]{article}

what does (Xe)LaTeX actually do/set?

I noticed that for example a multicols environment behaves different whether I use the [12pt], or no such option but a \fontsize{12pt}{14.5pt}\selectfont at the beginning of the document, with some spaces around headlines being 1 or 2 pt different. (The value of 14.5pt I got from \the\baselineskip from the original [12pt] version).

Answer 1

There are two major points here:

1. the option loads the file size12.clo which sets the lengths and sizes (not just the sizes for \small etc., but also paragraph indentation, the skip amount, apparently, a default for the margins, etc.

2. a package can assume font sizes are set that way by convention and look at the global options to make some changes in its own setup.