What is non-mainstream means that it is not the most popular and common in the same category.
For example, the mainstream CPU is Pentium, AMD, Celeron, and the highly anticipated Core 2Duo is too expensive for people to afford, and only a few people use it. This is the non-mainstream CPU.
Of course, mainstream and non-mainstream are not limited to price factors, but are mainly determined by the degree of audience and the number of users. Of course, this word can also be used in writing, consumer goods and many other fields.
In addition, we now generally use the word non-mainstream, mostly in dressing. It means to publicize personality, be different from others, not popular, and it means fashion trend.
In other words, fashion avant-garde is "personality", which suits the tastes of young people now. Non-mainstream has both good and bad aspects, which are mainly introduced below.