Tags are required for a certain types of token deposits such as XRP:

The tag is necessary for us to identify the transaction. Please remember to include it when you are initiating the deposit from other platforms or wallets. Otherwise, the transaction may not be credited.

A reminder will pop up if it is your first attempt to deposit tokens which requires a tag.

For the tokens without the tag displayed on the deposit page, it is not required for the transfer.

The same case applies for withdrawals. If you are withdrawing tokens to the platform which requires a tag, please remember to include it when initiating the transfer.

Remarks
You may enter anything in this field to help you identify the transfer.