When I talked about types of fighting infantry, I skipped two as they are less important for a newbie to care about. Those are militia and light infantry.
Militia is the cheapest type of infantry you can get. In return, it abandons both traits you want infantry for in the first place -infantry killing power and AT. The RPG sucks, the gun sucks. What do you want them for?
The only thing they excel at is HP – even with regulars, the cheapest you can get is 10 health for 15 points including transport. With militia, you can get 10 health for 10 points. Thus, they’re the best infantry to lead an open space crossing – it’s inevitable that the infantry doing the crossing will take some damage until it establishes a bridgehead into a forest or town, and militia minimizes the losses from that. Unfortunately bringing a card for just that job is too luxurious for most decks, and it requires a lot of foresight to know in advance where you’ll be doing such crossings, so militia is rarely used.
Light infantry is an upgrade similar to marines. You get 50% more AT ammo. This stacks with the marine bonus, with units being both marine and light bringing 12 shots. With one or two exceptions, light infantry moves 5 kmh faster than their training would imply – regular for example is 20kmh, but light regular is 25kmh. They tend to carry unique ATGM-style weapons despite being fighting infantry, but this too isn’t always the case. A unit being light usually means it will cost 5pt more than expected, or have restricted transport options (in extreme cases only coming in helos).
The light infantry type is too diverse and inconsistent to look at as a whole. Just look for the weirdly long-range AT weapon on an otherwise well-rounded squad, and consider employing the ones with lots of AP or without the [GUID] tag in defensive roles, particularly in cities. The long range+10hp combo is a big advantage as it allows you to keep firing and killing fire support even after the unit is spotted. The added MG compared to pure ATGM teams allows light infantry to fend off unsupported infantry pushes as well, making these very versatile defenders.