What does the font size documentclass option actually do?

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).

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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.
some document classes (for example `amsart` and siblings) have the size option code built into the `.cls` file. however, i know of no latex document class that departs from the basic model of `\normalsize`, `\large`, `\small`, etc., within the provided range of sizes. (the provided sizes of the ams classes are a superset of the sizes in the basic classes, adding `\Small` and `\SMALL`.) – barbara beeton Jan 18 '13 at 13:44