What is the difference between a 90m permanent link and a 100m channel?

The permanent link is the fixed installed cabling, normally from the patch-panel termination to the work-area outlet or another fixed termination, and is designed for a maximum of 90m.

The channel includes that fixed link plus the equipment and patch cords used at the ends. The conventional maximum channel length is 100m. Those figures are design limits; tester allowances for measurement uncertainty must not be treated as additional installation length.