Memo is optional. When you send tokens from an exchange to a personal wallet (Infinito Wallet, Cosmostation Mobile Wallet, Ledger, etc.), you do not have to insert a memo.
However, for exchanges that require a memo for deposits, you must insert the required memo or tag in the transaction.
If you forget to include a memo when transferring to an exchange, your deposit may not be reflected, and you must contact the exchange in order to resolve this issue.
Please note that Memo for ATOM in Infinito Wallet limits 100 characters.