FAA can issue two types of approval for a TSO part application: TSO Authorization and letter of TSO design approval. What's the difference?
Does it relate to the approval being design-only or production only?
Also, do other Airworthiness Authorities have a similar difference?
According to 14 CFR 21 Subpart O §21.601:
(2) A TSO authorization is an FAA design and production approval issued to the manufacturer of an article that has been found to meet a specific TSO;
(3) A letter of TSO design approval is an FAA design approval for an article that has been found to meet a specific TSO in accordance with the procedures of § 21.621;
(4) An article manufactured under a TSO authorization, an FAA letter of acceptance as described in § 21.613(b), or an article manufactured under a letter of TSO design approval described in § 21.621 is an approved article for the purpose of meeting the regulations of this chapter that require the article to be approved;