Many attorneys define their market by they do, as in "I am doing copyright" or "I am doing real estate". This is the continuation of university, where "law" was divided into "contract law", "commercial law", etc.
Another, client-centered way of defining your market is to define what clients you work for and what needs you meet for them. The two above examples will become "I help high-tech companies protect their intellectual capital" or "I help real estate developers to manage risks."
Some firms find it difficult though to define their market in this client-centered way, because every lawyer is doing something different. Of course, if the firm is a labour law boutique, defining the market is easy because the specific market of each attorney in the firm is the same as the firm's. But when in the same firms, each attorney is working for different clients in different areas to fulfill different needs, it becomes tricky to explain what the firm is about.
Time to be creative. What criterion to use? An example: the type of client relationship that you have. Or the pricing system. That is becoming the core feature of your firm and what links all lawyers together: a certain philosophy, a particular approach to client relationship, etc.
Antoine Henry de Frahan