A recap and slight elaboration on the various rules for hiding, plus rules for cover.
+10 AC if fully invisible, +7 AC if partially (e.g. body is invisible, but wearing some clothes). [MCC page 52, Holographic Skin] +2 to-hit [MCC page 124, Attack Modifiers, DCC page 78, same]. Interestingly, no mechanical effects of invisibility are defined in DCC, even in the Invisibility spell.
This covers alien powers and things like holo-suits or other active stealth technology.
Hiding behind things. DC for Hiding is the same as for Thieves: at night in the open or in dense foliage is DC 5; full moon in the open or moderate foliage in daylight is DC 10, etc. –1d if moving slowly, –2d if moving quickly. Remember that if you’re not an appropriate class or don’t have relevant background the roll is on a d10 instead of d20 before modifiers. A successful concealment roll means the enemy doesn’t know where you are until you start firing, and can’t target you, only the area you might be in (via area effect weapons or suppressive fire). Since DCC has no Perception checks, if you fail your hide roll you will be spotted unless the Judge decides the enemy are too busy or distracted to look, or gives you a pity Luck check.
Gives +2d on the Hide check (+1d if improvised) . Even if spotted, get +5 AC vs. missile fire as long as still within concealment. Requires at least light concealment, and carefully applied camouflage appropriate to surroundings, including camo-patterned clothing, face-paint, Ghillie suits, and the like. No bonus vs. area effect weapons or suppressive fire (as long as they’re targeting the right area). May be reduced or negated completely by higher Tech Level sensor gear.
Cover is any barrier sufficiently tough to stop missile attacks of the given Tech Level. Even a mound of dirt or fallen log might be sufficient to provide cover to a prone character, as long as the TL of the attacking weapon isn’t too high. If the target has full cover it can’t be hit; for partial cover figure out what fraction of the target is covered (1/4, 2/4, 3/4): roll to hit and if it’s a hit but not a critical roll a d4 and if the roll is less than or equal to the covered portion, then the shot hits the cover instead. (You could roll for cover before resolving whether it’s a hit, but this way saves you from having to roll if it’s a clean miss.)
Cover is also concealment, so if the cover is large enough you may be able to move under cover far enough that you can make a Hide check, and the enemy once again won’t know your exact position until you do something to give yourself away.
You can pop partially out of cover to take a shot without breaking cover, but you still take an automatic attack from any suppressive fire against the area, and any enemy that knows your position and has been specifically waiting for that also gets an attack. In these cases you gain no benefit from cover.