The CMIP6 experimental design and organization has been agreed at the WGCM 18th Session in October 2014, see details on the CMIP Panel website. Part of this covers the creation and timeline of the CMIP6 Data Request.
The data request is available through a repository, and the latest version is available here (updated November 6th, 2020):
An overview of the pressure levels proposed for atmospheric diagnostics is available for discussion (here).
Key documents describing the request (in the "docs" directory of the repository) are:
See Version 01.00.33 Release Notes for more details
Guidance on information request issued in December 2014 (updated, Jan. 16, 2015): CMIP6 Data Request Compilation Guidance Note
Data Request: A request from the CMIP panels to the modelling groups for data to be contributed to the archive, specifying the variables which are needed for each experiments.
Data Requirements: Documents specifying how the data should be delivered to the archive
MIP tables: tables which define the variables included in the data request. The "Data Request" and "Data Requirements" overlap, and the MIP tables are part of this intersection.