Its four pillars are the order-protection-rule (no trading through a better protected quote), the access rule (fair access and a cap on exchange take fees), the sub-penny rule, and the market data rules that created the consolidated feeds.
The consequence is the structure traders actually live in: dozens of venues, a stitched-together nbbo, mandatory routing logic, and a permanent argument about whether fragmentation helps or hurts investors.
Example: a broker holding a buy order cannot simply fill it on its own venue at 50.02 when another exchange displays a protected offer at 50.01. It must access the 50.01 first or route an intermarket-sweep-order that clears it. Without the rule, internalising at an inferior price would be free.
Related: order-protection-rule, nbbo, sub-penny-rule, reg-ats