There are many elaborations on this basic model:
- an entity between the sender and receiver catches the message and does something with it. The entity could cache it so future requests are faster, log that a communication occurred, change the message, completely block the message, or any number of other things.
- the communication medium itself may affect or manipulate the message somehow, either according to someone's intent or by accident (transmission flaw), especially as "the medium" can include computers and other such things.
There are any number of ways to expand on those basic elaborations of intercepting the message or manipulating the medium. An exhaustive list of such things would take a long time, but fortunately there is a relatively straightforward way of characterizing all of them. Each possible elaboration consists of some other entity inserting themselves into the connection somehow and manipulating the message between receiver and sender. For every communication made, we can catalog these entities and form a chain of responsibility.
- chain of responsibility
- The listing of all entities responsible for manipulating a message in some form other then pure delivery.
Consider a newspaper story containing a quote from some source and some commentary by a reporter. By giving the quote to the reporter acting on behalf of the newspaper, the source is trying to communicate with the readers of the newspaper story. In the communication from the source to the reader, the newspaper intervenes, probably edits the quote, and adds the rest of the article around it. In this case, the newspaper is on the chain of responsibility for this communication. This matches our intuition, namely that the newspaper could potentially distort or destroy the message as it pleases, and it has responsibilities that we commonly refer to as "journalistic ethics", which among other things means that newspaper shouldn't distort the message.
Being "responsible" for a message means that you are able to affect the message somehow, and are thereby at least partially responsible for the final outcome of the communication. As I write this, millions of conversations are occurring on a telephone. I am not on the chain of responsibility for any of those communications because I have no (practical) ability to affect any of those communications. On the other hand, if someone calls my house and leaves a message with me for my wife, I can affect whether that message is transmitted accurately, or indeed at all. Alas, too frequently I forget to deliver it. Thus, I am on the chain of responsibility for that message, since I have demonstrated the capability of destroying the message before it got to the recipient.
Anybody who can affect the message is therefore on the chain of responsibility even if they have no technical presence on the medium. The biggest, and perhaps only, example of this is a government, which may choose to set rules about all messages that affect all message and thereby have a degree of responsibility for all messages. For instance, the government makes rules about "libel" and "slander", and the government has the ultimate responsibility of enforcing them. Since a government is capable of censoring a message, they are technically on every message's chain of responsibility as a result, though the impact is so diffuse that usually as a practical matter it's not worth worrying about.
The chain of responsibility should consist only of people, corporations in their capacity as people, and "governments" considered as people. Any time it seems like a machine or process is on the chain of responsibility, it is really the person responsible for that machine or process who is on the chain. The reason for this is that a machine or device can not be "responsible" in the ethical sense for anything, since they are not people. Sometimes it is not obvious who that person is, and once again it can be a judgement call exactly who is responsible.
For example, consider a browser cache, which stores content from web servers on behalf of the browser user, so they don't need to reload it every time they wish to view it. Clearly "the browser cache" is on the chain of responsibility of a standard web page retrieval communication, because if it works incorrectly or has stale content, it can prevent the user from receiving the correct message. But since "the browser cache" is (part of) a program, and programs aren't allowed on the chain, who is responsible for the browser cache? The obvious answer is the browser manufacturer, but as long as the cache is implemented correctly, that is not necessarily the right answer. Consider a lawsuit against someone who took content from a web browser's cache and then illegally distributed it. Can the browser maker be said to be involved with this? One could make a case for it... perhaps the cache manufacturer should have encrypted the cached content better so the user couldn't just take it out of the cache. On the other hand, that's not necessarily a very convincing argument because clearly the person who is illegally distributing the content is responsible, and the fact that they got it from the cache merely incidental, as they could have just as easily gotten it directly from the website.
Presence on the chain is not a binary off/on thing, because there are different levels of responsibility. Sometimes it's not possible to strictly determine whether an entity is "on" or "off" the chain. As usual in the real world, there are grays, and thus there is room for legitimate disagreement about how responsible a given entity is in some situations. The most useful question to ask is "How much influence can the person have on the message?" Someone who can silently manipulate the message to say anything they please obviously has more responsibility then someone who can merely block one image from loading on a web page, and can't hide their responsibility. In the caching example, as long as the cache is correctly and honestly implemented, the browser manufacturer has no effective ability to control the messages I see.
Another example: We typically believe that the phone companies or Internet search providers should not examine or modify the messages we use their equipment to send, but just accurately transmit them. In other words, we expect the ISPs and phone companies to stay off the chain of responsibility by refusing to affect the message, despite the fact that physically, our messages travel via their equipment and they could fiddle with it if they chose. This is also a good example of a time when literal physical reality doesn't perfectly match our conception of ethics: If the phone company simply relayed our message, we do not hold them ethically responsible for the contents of the message, event though in a technical sense they are. In existing law we refer to this as "common carriers", entities that simply carry communication and are not responsible for the message, contingent on their not affecting the message.