All systems are necessarily closed. It has a range and limit. Everything within the system will define each and every other object within the system.
The English alphabet is one such system. There are 26 letters. each and every one of those letters contrast to each other for not being one another. Within the confines of these 26 letters all combinations of words are made. Saussure called this the system of difference. For the signifier this is difference is easy to understand. Together with the signified the story becomes less clear. Since the relationship between signifier and signified is arbitrary this means a signified can take any signifier. Up to a certain this can be true. However, the reality is that some signified meanings take on certain signifiers in the form of polysemy. Other forms of dictatorial tendencies may be seen in onomatopoeia, assonance and alliteration. In other words there is both arbitrariness and systematicity at work in the relationship between form and meaning.
To me, the implications of this is important to our understanding of the nature of language, and ultimately to the nature of thought.