Z39.50 is set up in the Customization Manager, allowing ILLiad to search for Local holdings you have.
We will need the Database Name (something like XYZ_INST), Server Address, and port number from the Database Provider.
Refer to: Connecting ILLiad and Koha Databases Using Z39.50. for completing the Z39.50 on Customization Manager.
Here are some links to the ILLiad Documentation on Z39.50:
If you update your settings for Z39.50, restart the ILLiad Client after saving the changes in the ILLiad Customization Manager.