You know how when you look at a topic, some fields are in the main area of the screen just near the image, and some are on the right hand side?

I used to think that anything that permitted multiple values went on the right, but it turns out that’s not so. Here’s the skinny on what’s really happening, thanks to Jeff:

You can actually specify which column a property appears in in the schema editor. Left is “horizontal” (because multiple values are displayed next to each other, separated by a comma), right is “vertical” (because multiple values are displayed above and below each other). The default layout is that properties for which the expected type is a primitive type (date/time, integer, text, etc.) display on the left, and everything else displays on the right.

(from this discussion about countries’ official languages.)

So, to summarise:

System primitives on the left.
Horizontal multiples on the left.
Vertical multiples on the right.
Non-primitives, even if single, on the right.