In case people reading this thread don’t know much about the various Churches in our great Catholic Church, nor what sui iurus means, allow me to provide this quick explanation:
There are many Churches in the Catholic Church; the Roman Church is the one most people associate with the Catholic Church, because they hear or say “Roman Catholic” all the time. But there are around two dozen others, including the Maronite Church, the Chaldean Church, the Syriac Church, the Coptic Church, etc.
In the Middle East, there is a Catholic presence, but it is not always easy for a Maronite Catholic to find the presence [that is, an actual church building] of the Maronite Church in their area, or a Syriac Catholic to find the presence [an actual church building] of the Syriac Church in their area. So sometimes they need to take advantage of the beautiful universality of our Catholic Church and attend the Divine Liturgy in a church belong to another branch of the one Catholic Church. This means sometimes a Maronite Catholic attends a Roman Catholic Mass, or a Syriac Catholic attends a Maronite Catholic Qurbono.
This is not against the law. They’re Catholics, so they can go to any Catholic liturgy and participate to their fullest ability. But they should not forget the “sui iurus” Church to which they belong!