Question: Relics are objects that are associated with saints and they are designated as either First Class or Second Class or Third Class. What is the difference determination between these three classes?
Answer: First Class relics are part of the body of the saint. Second Class relics are pieces of the clothing or something used by the saint. Third Class relics are items that have been touched to a First Class relic.
Read more
https://www.catholiceducation.org/en/culture/catholic-contributions/church-teaching-on-relics.html