"Henceforth" vs. "hereinafter"

They are both suitable, but the difference between them is that hereinafter (sometimes written as two words, herein after) usually pertains just to writing within a document, While henceforth is more general, and just means from now on. For what it's worth, I've only ever seen hereinafter in legal documents (my rent contract, most recently).

So, for example, you could say:

Henceforth, we shall not go to 7-11 for hot dogs.

but you could not say:

*Hereinafter, we shall not go to 7-11 for hot dogs.

Within the context of a document, either might be used, but hereinafter specifically limits itself to the document or corpus in which it appears. Therefore, it is often the preferable choice for coining a replacement term or phrase:

Hereinafter[?Henceforth], the hot dogs shall be referred to as exhibit B.

That does not mean that you cannot write henceforth in a document, however. For example, you could write:

In this thesis, I will support the idea that henceforth, all hot dogs should be called hot pigs.

Hereinafter would not be appropriate, since it places too much of a limit on the scope of the author's claim.

• "hereinafter": refers to a position in a document (from this point on; in a subsequent part of this document)
• "henceforth": refers to a moment in time (from now on; in the future)

So, to answer your question, "hereinafter" (or "herein after") is most likely what you'd be using for your purposes. Both are used only in legal documents, otherwise considered archaic.

It would be "ACME Corporation, hereinafter referred to as The Supplier" but "henceforth she shall be known as Mrs Miller".

Hereafter ("In time to come" and "From now on") works better, I think, than either of henceforth or hereinafter. For example:

When you (hereafter, the buyer) purchase this product, ...

Both words sound too 'legal'. If all you want to do is replace one word with another, you can just add it after the first use in parentheses, similar to what you would do with an abbreviation.

The United States (US) is a country.

The book I'm writing (my book) is about a dog.

Then, after that, you can refer to the US and my book, and the reader will know what you mean.