The link for this post uses a read-only front-end for Twitter, which can be easier to read for viewing a full Twitter thread. The Tweet that kicked off the thread is:
On July 13, Coinbase Exchange will be unifying USD and USDC order books. As part of the unification process, USDC order books will be merged under USD order books to create a better, more seamless trading experience with deeper liquidity for USD and USDC.