A blockchain is a protocol that allows transactions to be performed. An analogy in traditional banking would be SEPA and SWIFT. A blockchain allows assets to be created on it - cryptocurrencies, NFTs, tokens, etc. The same asset can sometimes be transferred between different blockchains. This means that the same cryptocurrency can exist on more than one blockchain. Usually, this happens with so-called “stablecoins” - cryptocurrencies whose value in fiat currencies is guaranteed by some company (for example, they ensure that 1 USDT coin = 1 USD).
Blockchains sometimes use the same standard for addresses. If blockchain A and blockchain B use the same standard, addresses from blockchain A would be accepted as valid by a wallet of blockchain B and vice versa. If a transaction is sent to the “correct” address but on the wrong blockchain, the funds can become inaccessible.
An analogy would be that every blockchain is like a huge building, full of unbreakable safes. Each safe is transparent, so everyone can see what’s inside each safe. Every safe has a slot, allowing anyone to put anything they want into the safe. Only the person who knows the safe combination can open the safe. If no one knows the combination, no one can open the safe. Most of the safes inside the building aren’t used by anyone, so no one can open them. When you send money to the wrong blockchain, it’s like if someone told you to deposit money to safe #123 in building A, but you accidentally deposited money to safe #123 in building B. Because no one owns safe #123 in building B, the money can’t be recovered (even though everyone can see that the money is there).
To ensure you don’t make this mistake, always double-check which blockchain your assets are on and which blockchain the receiver expects to receive assets.
In the case of CoinGate, we are the receiver, so on our part, we allow the shoppers to select which blockchain they want to pay on and/or inform the shoppers which blockchain the payment should be sent on. We have no way of knowing which blockchain the shoppers have assets on, and this information is provided by the wallet the shopper uses to make the transaction.