Roots or Brassicas

I’m getting confused as to what category turnips and swedes belong to. All the books say they belong to the brassica family yet some say they can be planted along with carrots and parsnips (roots). Does it really make much difference? 

They are brassicas, so you need to remember that so you don't plant brassicas there again the next year.

However in old allotment plans they were grown along with the other roots, as you say.

It doesn't matter where you grow them as long as you remember to not follow with more brassicas

snap!

I believe they are cucurbits/squash. I've read they can be planted anywhere and classed as 'other' but all books advise to try to include everything in crop rotation.

I am as above above, turnips/ swede go in my brassica bed and follow  the rotation. Squash and pumkin follow the peans/ beans around as I no longer grow sweetcorn they have taken there place.
For these I dig a biggish hole, fill it with rotted muck and back fill a little topsoil, leave for about a month then transplant into it, here is the tip of the day, when you put the muck in stick a cane in the middle so you know where your hole is.