Fashion does not define. It is instead a term that demands definition.
Fashion as the word is defined as a trend. It is a trend even if you are not “trendy.” The fact that it is a trend is inherently defining any given group of people, even if they look like they just crawled out of a hole. So fashions definition does demand definition into a kind of trend. Saying that fashion needs to be defined is reflective of its human quality. A person can’t live undefined or with no sense of purpose, even if you concede to have no purpose, that becomes your purpose. A clothing industry cannot be fully undefined because it would not hit its target consumer. Fashion has a general definition, but the industry is always seeking these and the consumer is looking for meaning in their choices usually reflecting the climate, both literally and figuratively, of society at the time
Fashion can be about confirmation, of self and others. But it is also about anxiety, ambiguity, and worry. As an aid to understanding psychological complexities, it is unsurpassed.
As in the above fashion, a trend is reflective of society. Society is complex due to the psyche of people, which people do not even fully comprehend. On a basic level, the trend you wear is chosen based on comfort or looking good for society. The latter is the complex part broken down to primal attraction to parts of the body and what many see as fashions whims, a far less graspable complexity. I agree with this statement, but I do think that often we layer to many complexities on fashion and art in the same way we use religion to understand your purpose.