To receive partial cover, a ’Mech must be adjacent to a hex one level higher than the level of the underlying hex it occupies, and that hex must lie between it and the attacking unit. The firing unit must also have an LOS level equal to or lower than the defending unit’s LOS level in order for the target to receive partial cover. In other words, an attacker firing downhill (regardless of how many hexes lie between attacker and target) negates its target’s partial cover.
The intervening obstruction providing partial cover must be a solid terrain type, such as a hill, a building, or DropShip. Bridges and woods do not provide partial cover.