Confirm the description about MTBF

Hi NV Team,

I found the Product Brief of Tesla V100 from below link.

https://images.nvidia.com/content/tesla/pdf/Tesla-V100-PCIe-Product-Brief.pdf

But I could not understand the means of “Uncontrolled” and “Controlled”. What is the difference in?

Uncontrolled environment: 1005907 hours at 35 °C
Controlled environment: 1869491 hours at 35 °C

Best regards.
Kaka