Once a Backer invests into a pool on the Goldfinch protocol, they receive a non-fungible token (NFT) that represents their investment into the pool. You can learn more about the function of that NFT in the Backers section of the documentation. It is important to note that this NFT could be freely transferable to any other Backer, such as by the implementation of backer secondary markets as outlined in the protocol's community-driven roadmap.
This could mean the Backers in your Borrower Pool could change during the life of the loan (e.g. backers trade their loans), although interacting with the Borrower Pool smart contracts will still have the same UID requirements.