BU Content refers to multimedia content that belongs to a fictional universe Big Hit Entertainment is creating as part of BTS' body of work (BU = BTS Universe). However, not everything from BTS is part of this universe, so relevant content is marked with either the BU Logo, or "BU Content certified by Big Hit Entertainment" at the end of Youtube video descriptions.
Chronologically, BU content dates back as far as mid-2015 with I Need U, but several videos were retroactively marked as part of the BU in mid-2017, when the BU was officially established in time with BTS' rebranding and the release of the WINGS concept book, which is where the BU Logo first appeared.
In fandom circles, the BU is sometimes still referred to as HYYH, which comes from the videos I Need U, Prologue, and Run. These videos were part of the album trilogy The Most Beautiful Moment in Life (kor. 화양연화, Hwa Yang Yeon Hwa → HYYH), and are now widely understood as the foundation of the BU, but the fandom already noticed they belonged to the same fictional universe before the official introduction of the BU, hence the fanmade name HYYH. Even Big Hit still uses HYYH in the name for HYYH the Notes (short: the Notes), which are notes about events on a specific date in the BU that are similar to diary entries and written by one of the seven characters.
The characters and their back-stories in the BU are fictional, but they borrow the names and age differences of the BTS members in real life. In the year 22, where so far most of the events in the BU take place, the characters are (most likely) between 17 and 22 years old.
A general warning: the BU story directly deals with potentially triggering themes like suicide, abuse, accidents, mental illnesses, and parental neglect. None of these themes are treated lightly or trivialized, but if you are generally sensitive to these topics, please proceed with caution! Specifically the webtoon is very graphic when it comes to death and injury. In the timeline there is an option to display trigger warnings.
For the sake of brevity this list only contains the longest/final versions of content that appears in multiple places (e.g. the Japanese Blood Sweat & Tears teaser is marked as BU Content, but only contains scenes from the MV, so only the MV is listed)
* Due to a glitch, some browsers don't display the "BU Content certified by Big Hit Entertainment" in the YouTube description, but it's there.
The Naver blog Testesso (translation) contains posts written by the owner of the fictional "Smeraldo" flower shop, a shop Seokjin encounters in the story (he also tweeted about the Smeraldo flower a couple of days before the Love Yourself era was introduced in 2017, which was when the blog was first discovered). The blog posts describe the history of the flower and the events leading up to the shop's opening. None of the BU characters are mentioned by name, but there are hints that the customer the owner talks about in the blog posts is Seokjin. And while this might be subject to change, up until now the blog doesn't really contribute to the plot itself, but merely provides some background information and context. Aditionally, the blog has only been active and changed/reuploaded the posts whenever BTS was gearing up to have a comeback that would introduce a new era (this was the case for the Love Yourself era (2017), the Map of the Soul era (2019), and the Dynamite single (2020)). So far the mentioned date of the flower shop's alleged opening always coincided with the time BTS had their comeback.
The twitter account Smeraldo Books was created in early 2019 to introduce the SAVE ME webtoon and the first book, HYYH The Notes 1, which contains most of the Notes from the albums Her, Tear, and Answer as well as new content. In August 2020 a second volume, HYYH The Notes 2, was announced, following a series of tweets with snippets from previously unseen Notes.
Some BU Content also makes very clear references to the BTS Begins concert VCR, and while we have no way of knowing if it's officially part of the BU because Big Hit cannot retroactively add a BU marker to this, it's safe to assume that it's at least partially relevant.
Song lyrics have never directly contributed to the BU storyline so far, if at all they only reflect the general mood or topic of a video.
The timeline is a chronological list of events in the BU, with information primarily taken from the Highlight Reel and the Notes, which are part of the physical package of the albums from the Love Yourself and Map of the Soul series, one set for each album plus a few additional notes uploaded on the official Big Hit Twitter. After the end of the Love Yourself era, a book was released that contained most of the Notes from the three albums, with several entirely new Notes and some changes in the old ones.
The purpose of this list is to structure and connect what we canonically know about the BU, without any further interpretations of the story apart from a few deductive notes about what's hidden directly in the text. It's meant to be an aid for those who want to understand what's going on, or want to draw their own conclusions.
Prior to the release of the Notes and the Highlight Reel (i.e. the start of the Love Yourself era) there was no information on which events happened when in the universe, and while the storyline is becoming clearer, there is still a lot of (video) content that currently can't be placed into the timeline. Seokjin time travels in an attempt to save the other characters, but for now it remains unconfirmed if the Notes describe events from different versions of the universe as a result of Seokjin's actions, and if yes, which Notes belong to which time loop.
The timeline doesn't include any events from the SAVE ME webtoon, since it shows multiple versions of how the story can play out on the same days, which would blow up the timeline considerably yet not contribute much to the understanding of the progression of the story as a whole in a chronological sense. It's best to read the webtoon in its entirety to learn more about the characters, and use the timeline as a guide, since the webtoon makes quite a few references to scenes from the Notes. However, none of the events in the Notes are ones that directly cause Seokjin to fail at saving everyone in the webtoon.
All notes contained in the HYYH the Notes 1 book will be added as soon as possible!