The Trusted Content Services is a pack of features that you can enable/purchase to gain more security for your Content Server. The main key features are listed in this documentation:

https://www.emc.com/collateral/software/data-sheet/h3138-security-ds.pdf

In our case, we wanted to benefit from the Repository Encryption feature:

Repository Encryption: Documentum TCS prevents intruders from accessing information even if they obtain unauthorized access to repository files at the file-system or storage level. This capability protects content against an operating system level security breach and enables you to securely store back-up media containing information assets in encrypted form.

To enable the TCS first you must get a license key from OpenText then they will send you a key String. Put that key only in a file located here: $DOCUMENTUM/dba/tcs_license

Then login to the docbase with IAPI and enable TCS for this repo with:

retrieve,c,dm_server_config
set,c,l,r_trusted_mode
1
save,c,l
reinit,c

No need to restart, the TCS license is only checked dynamically when a TCS feature is called.
To see if the license has been properly integrated, login to Documentum Administrator and on the Admin page you should see:
Trusted Mode: Enabled