Coaches still talk about a number six or a number ten, while the players wearing those shirts may play anywhere. The vocabulary outlived the system that produced it.
Numbers originally described the starting arrangement
When numbers were first used, they were allocated by position within a fixed formation, running from the goalkeeper through the defence and midfield to the forwards.
Because nearly every team used the same shape, the same number meant the same job everywhere, and a description was universally understood.
The system worked only as long as formations were uniform, and it began to break down as soon as they were not.
Squad numbering severed the connection
Fixed squad numbers were introduced so that players could be identified consistently across a season rather than by their role on a given day.
Once a number belonged to a person rather than a position, the informational content of the shirt disappeared entirely.
Players now choose numbers for personal reasons, and a striker wearing a low number carries no tactical meaning at all.
The vocabulary survived because the roles did
Coaching kept the old numbers as names for functions: the holding midfielder, the creator between the lines, the wide forward who cuts inside.
Those functions persist even where the shapes around them have changed, so a shorthand for them remains useful in a way the shirt itself is not.
The terms also travel between languages and countries more reliably than positional names, which vary considerably.
The shorthand has limits
Modern roles frequently combine functions that the old numbering treated as separate, which is why hybrid descriptions have appeared.
A full-back who moves into midfield in possession is doing two jobs that the original scheme assigned to different players, and no single number describes him.
Coaches respond by describing responsibilities directly rather than reaching for a number, particularly when instructing players who did not grow up with the convention.
What it still tells you about a squad
Number allocation is no longer tactical, but it is not entirely random either, since clubs often reserve certain numbers for senior players or for a position by tradition.
Academy graduates are frequently given high numbers on promotion and move to lower ones as they establish themselves, which makes the number a rough proxy for status.
Read that way it says something about a squad's hierarchy, which is a different piece of information from the one the system was designed to convey.