In an exchange like NYSE, with an actual trading floor, I understand that the floor, say, opens at 9:30am and ends at 4:00pm. These are the 'trading hours'. So,
After 4:00pm, there are after-hours trading with reduced liquidity, but what does this mean? Is it that most of the floor traders go hom,e but some stay back, and that is why liquidity is reduced?
For an online marketplace like ARCA, BATS, TSX, etc. how does the concept of trading hours vs. after-hours apply? Online means 24/7 usually, so how does this work?