I think what he means is that, by the time you apply for a TC, you should know:
(a) what type of work you want to do; and
(b) which part of the country you want to work in.
One of the questions which inevitably comes up in interviews is "Why do you want to work for us?". You then spout the usual garbage which you've gleaned from your research of that particular firm. Your answer is somewhat undermined if they find out you've also applied to a completely different type of firm at the other end of the country i.e. family law specialist Bloggs & Co in Stoke when you're sitting in an interview for Freshfields (and, indeed, vice versa).
Its fine if you've done vac schemes at a variety of different firms - that does show that you've researched the different areas of the law and practice. But applying to a massive variety for a TC, in a sort of scattergun approach, isn't impressive.