Remarks (tags or memo) is a special field when sending tokens of EOS. Creating EOS accounts cost much, so most centralized platforms such as exchanges will use a single EOS account to receive and assign unique ID to all the users, usually a number, to distinguish which user should be credited for the asset received. Therefore, Memo is required to transfer to a centralized exchange platform.
As for decentralized wallets, each account/address is owned by the user and is unique. So there is no need to fill in memo when sending tokens to a decentralized wallet. If the memo is mandatory when withdrawing from the platform, just fill in anything.
Attention: some wallets are also centralized, the same as exchanges.
Is Memo required when transferring from MYKEY to others?
Fill in according to the requirements of the transfer destination address. Generally speaking, you need to fill in when transferring to the EOS address of a centralized platform such as an exchange.
Is Memo required when transferring to MYKEY from others?
No need. You can write anything if required by the exchange.