It has to do with reach - billboards are typically high-volume, low-specificity impressions while placards and banners are typically event-specific and have a lower, more targeted audiences. There's gray area, obviously, like billboards in airports.

> but banners and placards on the roads are below the line

I would include that as above the line. My understanding is ATL is for mass viewing. While BTL directly or reasonably targeted to the individual. I say 'reasonably' as often EDM/DM (emails and mailers) have broad targeting cohorts & elements but are considered BTL.