Shirt-number shorthand for central midfield: the six screens in front of the defence, the eight plays ahead of him and joins the attack. It names the role, not the number on the back.
When a coach shouts that the six is dropping, he is not talking about a shirt number but about a job. The six is the central player right in front of the defence: he screens, distributes and, in build-up, often drops between or beside the centre-backs — that move turns a back four into a temporary back three. Two of them side by side make a double pivot. The eight starts further forward: he takes the lay-off in the half-space, turns and runs beyond. The most common misreading at amateur level is treating the six as a pure destroyer. Positioned well while your own team attacks, he is also the insurance policy against the counter — and the moment the ball is won back, he is the one with the whole pitch in front of him.