It matters. If you have a chance to rank and display in Local Search results for your primary category, personal injury in this case, you definitely want "personal injury attorney" selected as your primary category. If PI is selected as a secondary category it significantly decreases that page's ability to rank in the 3 Pack / Local Search results.
In this case I would probably make your secondary category Criminal Defense Attorney, then a 3rd category could be "law firm".
As with all things in SEO land, "it depends".