Unit groups

I m not good at these so how you use them? How many? Are like first 3 like temproary? Because I have difculties managing frontline specially if there is several frontlines and you want to avoid select all same type units.

Also I have idea which would improve groups like ctrl+1 selects whatever unit group and there is meta group with space + number which selects individual unit of that group in visible (or maybe not) range. So ctrl+number becomes like 10 20 30 and space + number becomes 11 12 13, or maybe there is something like this and I just dont know how to achieve this? Mostly I want select 1 bomber and give it target, "catch" it in air is quite hard and bind each bomber with ctrl looks weird too.

Also what space is used for and is it really used? For now I use it only when I want separate words in chat.

I ussually do CTRL+Z too, but often times I'll be moving units somewhere on the map where I should not be moving them and forget about them to find them running somewhere random later.

Is there any hotkey to select same units, but only on screen?

Wonderfull

This is how i use the unitgroups. They are pretty much setup the same as when i used to play Starcraft.

1-4 unit groups, these change constantly during the game. Often i assign groups just before a fight starts for easy micro. (some hot-key selections help with this )
Or
Every army on the field has a group and i try to micro with manual selection. (with the aid of the same key-selectors)

5-7 for factories. I'd really advise to assign every factory to a group and disable auto-grouping.

8,9 are bombers and gunnies Could do without those

In general unit groups are good for managing stuff which isn't in you view.