[Mobhack]

Smoke reduces visibility for each hex on a cumulative basis (only hexes with smoke values greater than a certain amount show smoke animations). So it is not a "wall" - each hex the LOS goes through contributes any obscuration it has currently, applies to all LOS checks, including TI for fire hexes.

Other than that - LOS is a two way thing. If A has LOS to B then (assuming equal vision capabilities) B can see A (eventually).

Since smoke in the target hex seems to be counted for spotting, then it may help a little bit in providing cover. Movement, firing and size would be more relevant factors.

There are no defence modifiers for a smoky hex.

Smoke is checked at the end of each player phase for drifting and dissipation, so you cannot rely on it as something to hide behind. A wisp of smoke may just have provided a LOS block for your tank in your move, but drift away at the end of your turn, revealing him to the enemy. Or or may drift further an block the LOS line your ambush was relying on etc.

Andy