Explanation:

Sale or Return Day Book and Sale or Return Ledger are both types of memorandum books.

Memorandum books are subsidiary books that are used to record transactions temporarily before they are posted to the respective ledger accounts. These books serve as a memory aid for the bookkeeper or accountant and help in maintaining accurate records.

The purpose of a Sale or Return Day Book is to record the details of goods sold on a sale or return basis. When goods are sold on a sale or return basis, the ownership of the goods is transferred to the buyer, but the buyer has the option to return the goods if they are unsold or not as per their requirement. The Sale or Return Day Book helps in keeping track of such transactions, including the date of sale, the name of the customer, the quantity and value of goods sold, and any returns made by the customer.

The Sale or Return Ledger is used to record the details of individual customer accounts related to sale or return transactions. It contains the customer's name, the amount of goods sold on a sale or return basis, the amount returned, and the balance due from the customer. It helps in monitoring the outstanding amounts and managing the accounts receivable for sale or return transactions.

Principal books are the main books of accounts that include the Cash Book, Journal, Ledger, and Trial Balance, among others. These books directly record the financial transactions and form the basis for preparing the final accounts.

Subsidiary books are books of accounts that are used to record specific types of transactions or categories of accounts. They provide detailed information and are later summarized and posted to the respective ledger accounts. Examples of subsidiary books include the Purchase Book, Sales Book, Cash Book, and Petty Cash Book.

Therefore, since Sale or Return Day Book and Sale or Return Ledger are used to temporarily record specific types of transactions, they are classified as memorandum books.