Why are some Christian churches called "ecclesial communities"?

How is it that Rome can claim to be “church” while everyone else is simply an “ecclesial community”?

Magisterial documents use the term church for the Catholic Church and Christian denominations that maintain a valid apostolic succession (e.g., Eastern Orthodoxy). Those Christian denominations that have not maintained valid apostolic succession are called ecclesial communities. The purpose is merely to distinguish between groups that have maintained historic ties to the original apostles through apostolic succession and those that have not; it is not intended to be derogatory toward non-apostolic Christian denominations.

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