I don't think specialization in this context means framework or even language. Someone who is really good at JavaScript is of very little use as a consultant fo

I don't think specialization in this context means framework or even language. Someone who is really good at JavaScript is of very little use as a consultant for a business. They want someone who knows the business. So I think specialization in this context either means knowing a lot about the infrastructure - e.g. being able to do whatever your client needs in AWS quickly and in a cost-effective manner - or, knowing a lot about the business domain, whether that's healthcare, logistics, etc. The code is usually a byproduct of that and being an "expert" in a JavaScript framework isn't going to sell anything to a hospital system looking for a software consultant.