In football, or soccer, if you prefer, the traditional significance of particular numbers comes from practical reasons. In the present, these are no longer a factor, and as a result player numbers vary wildly with few, if any, limitations. Still, football is a traditional sport and a degree of this remains to this day – for example, you will see a striker being referred to as "a traditional number nine", or defenders often playing with lower numbers.

Generally speaking, shirt numbers have reflected the change in players' position on the playing field. When they were first issued in Europe in the 1920s, teams would usually line up with a goalkeeper, two defenders called fullbacks, three midfielders known as half-backs, and five forward players (with a variety of names). Numbers would then be assigned from back to front, so to speak. That's how the two defenders got to wear numbers 2 and 3, the half-backs 4, 5 and 6, and the strikers wore the rest, with 9 being the centre forward.

As time went by, formations and tactics would change. For example, the two fullbacks would be joined by a third and fourth defender withdrawn from the half-back line. The fullbacks would be then pushed wide – hence the term 'fullback' for the wide defenders in a team's defensive four used to this day. This also explains how these players came to wear the numbers 2 or 3, or why central defenders in England often used to be numbers 5 and 6, as they originally came from midfield.

When it comes to the forward players, the changes haven't been that pronounced, although we no longer talk about having five forwards, as most of them have been moved deeper and deeper into the middle of the pitch over the years. I already mentioned the number 9 being the centre forward in the five-player attack, meaning he was the most central of the attacking players and the main goal threat. You can see how a tradition of the "number nine" as a striker, a goalscorer, would develop.

Meanwhile, 7 and 11 were the wide players, something you can still see to this day fairly often. Players wearing 8 and 10 used to be called inside forwards. They played not quite on the wings, nor entirely up front. In simple terms or in today's language you might call them attacking midfielders, and indeed those are the players who have often wore these numbers when formations started to change to the shapes more familiar to us in the present.

Either one or both of these players would be pushed into the midfield. It was usually the number 8 in this deeper role, hence the relative prominence of this number for what we today label as central midfielders. Whereas 10, a magical number on its own regardless of the context of football, has often been issued to the second striker – if he did not get withdrawn to the midfield – or the glamorous playmaker, the star player, or simply the more attacking of the original inside forward duo when their position on the field shifted.

There are regional and cultural differences to this, of course. For example in England the number 6 is often associated with central defenders, while in Germany it is a traditional number of the more defensively minded central midfielders. Brazil and South America in general also has a slightly different shirt number tradition due to how the sport, or rather the shape of its lineup, developed there. If you are interested in a dense but quite interesting take on the development of the tactical side of football, which inevitably ties into its traditions and all of the above, I have to recommend the much spoken about and vaunted (but ultimately deservedly so) book Inverting the Pyramid.

I can't provide any context, but only an example, I hope it still can help you!

In rugby, a player can not chose his number. the number on his back just stands for the position he's playing as. Here's an image that sums it up perfectly.

EDIT : Can -> Can't