Pro Tools DigiLink I/O License for Pro Tools HD 12.6 through Pro Tools Ultimate 2021.12 - not required as of Ultimate 2022.4 anymore

The Pro Tools DigiLink I/O is a license that unlocks additional input/output (I/O) channels for Pro Tools | HDX and HD Native systems. It allows you to use up to 192 channels of I/O with Pro Tools | HDX (with 3 cards) or up to 64 channels with Pro Tools | HD Native. This is double the number of channels that you can use with Core Audio or ASIO. 

Download: https://www.avid.com/redemption