Not all transactions are transfers.
In transaction request, A Transaction is created like for example minting a NFT and any possible tx on solana. This transaction is read by wallet's then they parse the transaction and ask's the user to sign it from their phones.You can better understand it's working and implementation from this docs here and transfer request's are simply used to transfer x amount of SOL or SPL tokens from user's account to a merchant account. Transfer requests are kind of subset of Transaction requests.
