Monero (XMR), Ripple (XRP), Binance Coin (BNB) and NEM (XEM) developers implemented extra fields for transactions, other than just destination and value. They are "Payment ID", "Destination Tag", "Invoice ID", "Memo" and "Public message", respectively.
The use of these fields is not necessary if you are receiving those coins in Coinomi. Likewise, they aren't mandatory for normal transactions when sending to your friends that also use Coinomi. You may wish to ask someone to add a payment ID or destination tag purely for easy identification of incoming payments. It is completely optional for a transaction to have these when you are receiving in Coinomi. If the use of a tag is mandatory when withdrawing from an exchange, you can use any value. (Typically a 9-digit value)
When sending to exchanges or payment processors they may require that you use the correct value when depositing. Not only the destination address must be the correct one, but the extra field (Payment ID, etc) must also be the exact one provided by the exchange. This is because it may be less costly for them to have a single deposit address for all users. So in order to know which deposits should be credited to which user, a unique value generated for each user that must be informed using those extra fields.
Once coins are sent and the transaction is confirmed on the blockchain, it is irreversible. When depositing into exchanges make sure that you enter all values exactly as they are required. Failure to do so may cause your funds to be irretrievable since the exchange won't be able to credit your account for the deposit. If you sent funds to an exchange that requires a memo/tag but failed to use the correct value, please contact their support.