URIs embedded in QR-codes, hyperlinks in web-pages, emails or chat messages provide for robust cross-application signaling between very loosely coupled applications. A standardized URI format allows for instant invocation of the user's preferred wallet application.
`prefix` is optional and defines the use-case for this URI. If no prefix is given: "pay-" is assumed to be concise and ensure backward compatibility to ERC-67. When the prefix is omitted, the payload must start with `0x`. Also prefixes must not start with `0x`. So starting with `0x` can be used as a clear signal that there is no prefix.
`payload` is mandatory and the content depends on the prefix. Structuring of the content is defined in the ERC for the specific use-case and not in the scope of this document. One example is ERC-681 for the pay- prefix.
The need for this ERC emerged when refining ERC-681. We need a container that does not carry the weight of the use-cases. ERC-67 was the first attempt on defining Ethereum-URIs. This ERC tries to keep backward compatibility and not break existing things. This means ERC-67 URIs should still be valid and readable. Only if the prefix feature is used, ERC-67 parsers might break. No way was seen to avoid this and innovate on the same time. This is also the reason this open prefix approach was chosen to being able to adopt to future use-cases and not block the whole "ethereum:" scheme for a limited set of use-cases that existed at the time of writing this.
