This list constraint defines one or more Cloud Identity or Google Workspace customer IDs whose principals can be added to IAM policies.
By default, all user identities are allowed to be added to IAM policies.
If this constraint is active, only principals that belong to the allowed customer IDs can be added to IAM policies.

Affected resources:

GCP IAM

In Solvay...

All users accessing the GCP projects are conformed to only use solvay.com account.
This is to ensure users accessing the GCP projects are adhering to onboarding/offboarding process.

If external/contractors no longer serving Solvay's mission, they will be offboarded or clean up by the process.
You will not need to worry if the users are still within the GCP projects or did the other companies properly disabled their accounts.

Unfortunately no exception will be entertained.
Exception is mandatory only for PO2.