So let's get a group of guys² together, friends already if you will. Now they pick up this 'game' and are going to try it. One, fairly motivated guy is going to teach it to them as they play. Beyond that, it's just an average get together to 'hang out'.
This is your basic social situation and the game could be anything.
But let's say it's a role-playing 'game'. The motivated guy will be the gamemaster for the first few attempts. They go through a process of assimilation of the 'game' and how each will interact with it.³ Meanwhile, the become superficially familiar with what they can expect from the game and more importantly, as a group, determine how their group will view those expectations. Then they are playing.
As they play, they habituate this ritual of play. I would argue that no two social groups are identical in the habits around these rituals. This is a very important point! Imagine one player playing in two groups using the same 'game'. Even though the texts upon which group expectations are formed are identical, the rituals can be very different.
Understanding this leads to very complicated problems for the writer and/or publisher position as theorist. This comes from the degree which the creator of a game can actually affect the final ritual structures employed by any group. As the finished 'game' is the practical limitation of the reach of its creator, it's excusable if they theorize primarily from a 'system' point of view. What has to be said is that it is an almost cripplingly incomplete theoretical center.
It should be clear that how the group assimilates the product or 'system' into their ritual of play is the real crux of any theory. The end supplied by the creator is very important but in no way dwarfs the affect of the social situation as the ritual is formed. I could argue that the social aspect is the larger, but leave that for another time.
It could be possible to co-opt these rituals into the concept of 'system', but that makes it almost impossible to speak effectively on any such structures on the terms which the creator can affect. In other words, combining these rituals into the meaning of 'system' makes the word 'system' mostly meaningless as a theoretical entity.
As an example of the difference, I would suggest the reader consider how similar their experiences of the rituals of play are in the same group using different 'games'. I have only heard of a handful of individuals who find these more different than they are similar. However, this is purely anecdotal and therefore restricted to being only a feature of my own opinion.
Or so I would like you to understand.
¹ Intentional mispeeling
² Unfortunately true, hopefully for not much longer
³ Sometimes known as Chara generation
p.s. I am going to be starting a 'gamemaster advice' only relatively soon. Look here to be the first to know!