This CAT Alert describes how Industry Members must transition from Phase 2c to Phase 2d data submissions and the Coordinated Industry Test Plan for Phase 2d. This CAT Alert also describes how FINRA CAT will provide support during the transition period between Phase 2c and Phase 2d. This CAT Alert also discusses how FINRA CAT, LLC will facilitate six coordinated industry tests with the first test commencing with CAT Trading Day September 10, 2021.