SEC XBRL financial filings are representations, not presentations, of the information they contain. Many accountants see these filings as presentations. They look at the XBRL as simply the Word document or HTML version of the report with the different pieces of the report "tagged" with XBRL. In fact I have seen training material which shows exactly this, usually written by accountants who likewise don't really grasp XBRL.
And I guess I should not be surprised by this view of XBRL because there is even software which works in this manner, "tagging the presentation" in order to fit the completed Word or HTML presentation into XBRL.
This presentation orientation is not appropriate. SEC XBRL financial filings do have a presentation relations perspective. But they also has a calculations relations perspective and what is called a definitions relations perspective. Now, don't confuse the linkbases or XBRL technical syntax with the meaning or semantics and don't confuse the explicit semantics of XBRL Dimensions and the implicit semantics when XBRL Dimensions is not used. These are complex conversations and I don't want to get into a deep discussion about that; I do want to make two points.
The first point is that there are three different relations, not one. That alone provides enough evidence to show that XBRL is not a presentation. Sure, presentation is part of the representation but it is only part and in XBRL it is a very weak part of the representation. For a long time I have pointed out that the XBRL calculations are impotent because they cannot be used to express all the computations within an SEC XBRL financial filing. The presentation relations are even more impotent, when it comes to expressing a presentation a user would expect for an SEC XBRL financial filing.
The second point is the use of the term "tag". There are a lot of reasons using the term "tag" is a bad idea. First off, the term "tag" is at best a syntax term many people associate with XML. XBRL really has no "tag" from the syntax perspective and it certainly does not have tags from the semantics perspective. But let's pretend I am wrong and say that tags do exist (even though they don't). What does the tag represent? A network? A [Table]? An [Axis]? A [Member], [Line Items], a concept, an abstract concept, the reported fact itself, an attribute of the fact such as the units or number of decimals? Hopefully you see my point. The term "tag" is too general even if the term did exist. Fact is, you could stretch things and say that a tag exists from the XBRL technical syntax perspective, but from a semantics or meaning perspective the term means nothing.
An SEC XBRL financial filing is a representation, or model, that a computer can understand. It is true that the XBRL technical syntax "delivers" or "exposes" the information to a computer, but it is the semantics or meaning which the computer is trying to understand.
There are two "layers" of meaning or semantics, it seems to me, when it comes to an SEC XBRL financial filing. The first layer is the business reporting layer which uses things such as networks, [Table]s and [Axis] to break up the components of a filing into discrete pieces of the model or representation. The second layer is the financial reporting semantics of the filings: does the balance sheet contain assets, does the balance sheet contain liabilities and equity, does assets equal liabilities and equity, does assets foot, does liabilities foot, and on and on; all those semantics which accountants know exist within a financial report.
Your representation, your model needs to work. Your model may be different that some other SEC filers model because, say, you are a bank and they are an airline. This is not a problem for XBRL to express in fact that is a core strength of XBRL, its flexibility to do just that. But again, each of those models need to work, for the bank and for the airline, and the core semantics of both models should agree where they should agree.
Does a bank have assets? Sure. Does an airline have assets? You bet. Does assets = liabilities and equity? Does assets foot?
That is what your representation, your model, of your SEC XBRL financial filing is all about.