You can categorise them as competitors by a few different ways - size of firm, specialisms (because a boutique place is a little powerhouse could be a competitor in a certain practice area), client size/profit rates within the sectors the firm services. A good way is to go to the firm’s page on legal 500 and click on a specialism they have then see who else is ranked similarly to them and then check if they do similar work within that practice area. Hope this helps ☺️