The six-digit look is the most recognisable thing about gallery references, and it is a display convention rather than a format. Underneath it is an ordinary counter: the first upload gets one, the next gets two, and the number is shown padded to a fixed width. Everything people find mysterious about these references falls out of that one unremarkable design decision.
A counter in a fixed-width costume
Sequential integer identifiers are the default in almost every database, because they are cheap to generate and guaranteed unique. Displaying them padded with leading zeros makes them look uniform, which reads as a code to anyone who has not thought about it.
The padding is cosmetic. The underlying value is just a count of how many items have ever been created, which is why a low number means an early item and a high one means a recent one.
This is the same design in a shop order number or a support ticket. Nothing about it is specific to this field, which is why the resulting shorthand behaves exactly like every other identifier shorthand, as discussed on the page about gallery codes.
Gaps, deletions, and why the count lies
Numbers are consumed even when an item never becomes public. Failed uploads, duplicates, and removals all take an identifier with them, and that identifier is never reissued. As a result, the highest number in a library is always larger than the number of items it holds, sometimes substantially.
This matters for anyone reasoning about scale from a reference. A high number tells you how many upload attempts have occurred, not how much is available, and the gap between those two figures widens every year.
Reissuing an identifier would be far worse, because every stored reference to the deleted item would suddenly resolve to something unrelated. Permanent gaps are the correct choice and they are why the numbers behave the way they do.
What happens when the digits grow
A counter does not respect a display width. When a library passes the point where six digits are enough, it starts producing seven, and anything that assumed six breaks. Scripts, bookmarklets, and community habits all have to adjust, usually after something has already failed.
Libraries usually see it coming and do nothing, because there is no clean fix. Padding to seven digits early makes every existing reference look wrong; waiting means a transition period where both widths circulate. Either way the community absorbs a small permanent inconsistency.
It also breaks the visual convention. Part of why the shorthand spread was that references were instantly recognisable at a glance, and a mixed-width set is less so. Small formatting details do a surprising amount of work in making a vocabulary spread.
What a number cannot encode
Sequential identifiers hold exactly one piece of information: order. There is no room in them for content, artist, language, or version, and adding meaning to identifier digits is a design mistake in any system because it breaks the moment the meaning changes.
That is why the description has to live elsewhere, in a title and an artist name. The comparison between the three reference types is on codes, tags, and titles.
Why the design is fragile for readers
An identifier is meaningful only inside the system that issued it. It is not a name for the work; it is a name for a row. If the library restructures, migrates, or closes, the row is gone and every reference to it becomes a number with nowhere to point.
That fragility is not a flaw in the database design, which is doing its job correctly. It is a flaw in using an internal identifier as a public reference, which is what the community did, as covered on the page about link rot.
What the digits are really telling you
Roughly when something was uploaded, and nothing else. Not what it is, not who made it, not whether it still exists. Read that way, the format stops looking like a code and starts looking like what it is, which is a row number in a costume.
The habits worth keeping alongside it are the same as ever: record a title and an artist, and hold to fictional adults, unmistakably grown, with credited makers. That is on the understanding of this house.