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: Note: The following is an excerpt from the official Ethereum site. It is provided here as a convenience and is not authoritative. Refer to the original document as the authoritative reference.

TransactionType || TransactionPayload is a valid transaction and TransactionType || ReceiptPayload is a valid transaction receipt where TransactionType identifies the format of the transaction and *Payload is the transaction/receipt contents, which are defined in future EIPs. Motivation

In the past, when we have wanted to add new transaction types we have had to ensure they were backward compatible with all other transactions, meaning that you could differentiate them based only on the encoded payload, and it was not possible to have a transaction that matched both types. This was seen in EIP 155: Simple replay attack protection where the new value was bit-packed into one of the encoded fields. There are multiple proposals in the discussion that define new transaction types such as one that allows EOA accounts to execute code directly within their context, one that enables someone besides msg.sender to pay for gas, and proposals related to layer 1 multi-sig transactions. These all need to be defined in a way that is mutually compatible, which quickly becomes burdensome to EIP authors and to clients who now have to follow complex rules for differentiating transaction types.

By introducing an envelope transaction type, we only need to ensure backward compatibility with existing transactions and from then on we just need to solve the much simpler problem of ensuring there is no numbering conflict between TransactionTypes.