Formations are written as three or four numbers, and those numbers describe how a team lines up when it is defending. In possession the arrangement is usually something else entirely.
The notation comes from the defensive phase
The numbers count players in horizontal lines from the back, which is a meaningful description only when the team is organised in lines, and that happens when it is out of possession.
With the ball, players move into positions determined by where the space is, and the neat lines dissolve within a few passes.
A team described as playing four at the back may spend most of its attacking time with three defenders and an extra midfielder, without ever changing formation in any formal sense.
Two shapes are more useful than one
Analysts increasingly describe teams with a pair of shapes, one in possession and one without, because the difference between them is where the tactical intent lives.
A side that defends in a flat block of four and four but attacks with wing-backs high and a midfielder dropping between the centre-backs is running two structures, not one.
The transitions between those two arrangements are where teams are most vulnerable, which is why the movement is rehearsed as carefully as the shapes themselves.
The same numbers cover very different teams
Two sides described identically can play in opposite ways depending on where the block sits, how aggressively it presses, and whether the wide players defend or attack first.
A high-pressing arrangement and a deep, compact one may be written the same way while producing entirely different matches.
The description is therefore a starting point for analysis rather than a conclusion, and coaches rarely regard the numbers as informative on their own.
Roles matter more than positions
What a player is asked to do within a shape varies more than the shape itself, and two players occupying nominally the same position can have unrelated jobs.
One wide midfielder may be instructed to hold the touchline and stay high, another to tuck inside and support the central midfielders defensively.
Because the notation cannot capture that, coaches describe their teams in terms of principles and responsibilities rather than in numbers.
The notation persists because it is convenient
Despite its limits, the shorthand survives because it communicates a rough picture quickly, and because supporters and media need a common vocabulary.
Its main risk is that it invites debate about which formation is best, a question that has no answer independent of the players available.
Read as a description of the defensive starting point rather than as a philosophy, it remains a reasonable piece of information and a poor summary.