David Foster Wallace makes extensive use of Footnotes and Endnotes in his works.
Out of the two, footnotes are the most common way that Wallace inserts more information on a topic without having to put it in the main text. These appear, as indicated by the name, at the foot, or end of a page and are numbered, generally starting with 1, from the beginning of chapters or sections. For the most part, the footnotes are helpful clarifications or interesting sidenotes. However, there are two extreme modes that Wallace falls into when using footnotes.
The first is that the number of footnotes begins to get out of hand. One example of the first mode is his use of 128 separate footnotes in the last section of Everything and More. This averages out to almost 2 footnotes per page throughout the whole section. Everything and More is probably Wallace's most heavily footnoted work, which could be a result of its inclusion in a series of books as opposed to a standalone piece by Wallace.
The second mode is when Wallace's footnotes generally begin to take over whole pages of the works. These footnotes even have a tendency to spill over into the following page(s) at some points. Unlike the first mode though, these long footnotes have information that is actually very important to understanding the story as a whole.
One of the worst (best?) examples of this is in the story Octet, in Brief Interviews with Hideous Men. Wallace uses some of the longer footnotes in this story in order to break the fourth wall so to speak and address the reader directly. In the Octet footnotes, he also uses the space to discuss how he failed to write the story he originally intended. Along with this, he includes stories in the footnotes that he started the footnote by saying he threw out.
In some cases, the footnotes themselves begin to have footnotes, which leads to a deep structure of layers upon layers in his works.
This abbreviation is short for "If You're Interested" and is used exclusively in Everything and More. Wallace uses this in order to try to indicate to the reader which footnotes are necessary to read in order to fully understand what he is saying and which are not. The abbreviations spills over into more than just the footnotes as the work continues, to the point of labeling whole sections of text as IYI. In this case, these sections no longer merely exist at the bottom of the page and can be breezed over, but stand directly in the path of the reader.
With this in mind, if Wallace has to indicate that some sections/footnotes are necessary and others aren't, it brings up the question: "Are all footnotes that he includes necessary for understanding his other works?"
One theory is that they are all necessary to the complete understanding of the work, but the publisher for Everything and More required Wallace to try to pare the footnotes down for a more general audience. It could also be that none of the footnotes are necessary. In this case, the question "Why does he include them at all?" rears its ugly head.
Out of all of Wallace's works, the only one that utilizes endnotes is Infinite Jest. The endnotes in IJ start at 1 (with a reference to meth) at the beginning of the novel and end at 388 (with a reference to another drug, Talwin) on the third to last page. Most of these endnotes are merely footnotes that have been relocated and collected in the back of the novel. Others however, have no business being called "endnotes," as they go on for pages and pages at a time--and in an excrutiatingly tiny font at that. Wallace uses these prolonged endnotes to add a second narrative. These endnotes, rather than make the reader's job easier (as shorter endnotes do by quickly explaining a reference), they make the reader's job that much harder, as they further dilate and obfuscate the already convoluted narrative.
The endnotes allow for simultaneous sub-narratives, somewhat external to the text proper, and this allows Wallace to both assert narrative authority, as he controls the endnotes and places them where he sees fit; but also to subvert that narrative authority, especially with the longer endnotes, which draw the reader away from the text for extended periods of time, and editorializing endnotes, which demand the reader seriously question who the narrator or authoritative voice is.
These sub-narrative endnotes are often chapter length (or longer) chunks of text which go into detail about certain portions of the narrative. The first of these huge endnotes is a complete description of Himself's filmography and takes approximately 10 pages to fully detail each film Himself made. At the end of this endnote, we get a description of Infinite Jest (film). One of the more interesting endnotes is number 304, which is a history of the AFR, plagiarized by Struck, from a random source. Another describes the story of Pemulis being kicked out of ETA. A endnote (no 324) is inserted into a space between two chunks of text on page 787. When we flip to the back, we find that Wallace has placed a whole chapter about Pemulis there. The end of the story is revealed in endnote 332, introduced in the same way as 324, a few pages later on p. 795.
Not satisfied with just tucking these narratives away at the end, Wallace goes another level deeper with his endnotes in Infinite Jest when he starts adding footnotes to the endnotes.
In Infinite Jest, the process of flipping between the text and endnotes provides a physical example of Lyle's advice never to pull more than you weigh. The act of heaving the pages back and forth, back and forth, is very much like the bricklayer being pulled by the pulley's weight. As a reader, one could opt of of the endnotes, but if one is commited to the text, one must flip, and is thus pulled along by this weight. This is especially true in the first half of the novel, when the amount of pages one must flip exceeds the number one has read. The more one reads, however, the more one understands about the novel, and the less weight one has to grapple with to access the endnotes. The act of reading Infinite Jest is marked by profound introspection and effort on the reader's part, and it is a much-needed physical relief when the burden eases.