what the meaning of receiver and code of an apply function

[breadbread1984]

is the receiver the message receiver? is the code the message sender? If so, I am confused that the implement of EOSIO_ABI requires receiver == code to process the message. then only the contract account itself can send message to itself.

[wanderingbort]

when an action is sent to the network, it specifically addresses a single contract account (the "code" account) and an action on that account. However, the contract can use require_recipient to notify another account of the action so that a contract on that account may respond. When it does this, it does not change the "code" account. This resolves any ambiguity between receiving the action "myaction" as a relay from another contract "notme" vs directly to your account "me".

In these cases
initial action is delivered to "notme" with code and receiver == "notme" then redelivered to "me" with code == "notme" and receiver == "me".

receiver should always be the name of the account the contract is running on however, as it is possible to upload the same contract to multiple accounts it is convenient to have this passed in at runtime.