Committment ceremonies are for a couple who wish to show their commitment to one another in front of their families and friends. Although they can often resemble a wedding, the ceremony is not legal and there are no legal documents to complete. This ceremony is suitable for same sex couples or couples who don't want a legal ceremony.
Re Affirmations are for couples who are already legally married and wish to renew their vows. This is often done when there is a milestone anniversary or if the couple has been married overseas or in another religious ceremony. eg traditional Hindu.
This ceremony also resembles a wedding but without the legal element.
Please call me if you have any questions.