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Noob question ... the little I, II, II, IV, etc. ... what are they for?

I assume they're just numbers for the unit, like "unit 1", "unit 2", etc., where all the models in any given unit get the same number. Or?

If there's anything else I should know to get the decals "right", then please leave your tips here.

You hit the nail on the head. They're unit markings!

When dealing with space marines, codex compliant chapters will have their appropriate unit designation on the right shoulder pad and right knee. 1 thru 6 for tactical squads, 7 & 8 for assault squads and 9 &10 for devastators. Or whatever the new primaris equivalent is, I'm not sure about them.
Some chapters (Ultramarine, Imperial Fists, etc) will use the Roman numerals. Other chapters (Dark Angels for example) will use the numbers in the same way.

And you can use them for the chapter numbers Bloodangels have the number IX.