Coinbase Pro just tweeted to say that ZRX trading is now available on all three books. The ZRX/USD, ZRX/BTC, and ZRX/EUR order books are now in limit-only mode. Limit orders can be placed and matches may occur. Market orders cannot be submitted. Order books will remain in limit-only mode for a minimum of 10 minutes.
Before, Coinbase Pro said that the trading on ZRX/USD, ZRX/EUR and ZRX/BTC order books has entered Post-Only Mode. Customers can post limit orders but there will be no matches (completed orders). Order books will be in post-only mode for a minimum of 1 minute.