senseo, those are the multipliers applied to small arms firing at trenches. technically they are unique to each weapon, but the vast majority of small arms have identical cover tables.
rofltehcat makes a good point though. the units technically are garrisoned in the trench. its possible they only recieve a garrison bonus, not the trench bonus. i did say i havent looked into this much

but there are trenches in multiplayer. not "non-doctrinal" but prebuilt on the map. city 17 winter has a lot of them. steppes has a couple by the center fuel. units dont garrison in these though, so its very possible that rofltehcat may be on to something.