The CR1224t is a CO2 release cover for all 24-deepwell MTPs. The holes in the cover are so small, that passive diffusion is negligible, but pressure, for example due to the formation of CO2, can not build up.
Other characteristics of the covers:
The sandwich cover its function relies on the vertical force applied by a clamping system such as CR1801h or CR1700
Autoclavable and thus reusable many times
The cover has a blue and green filter to prevent airborne contamination
The silicon layer is made from very durable rubber, which is free of any animal product