Shulchan Aruch, OC 212:1 (with my own loose translation):
All that is main, and with it is a subordinate (tafel) thing — one says a benediction on the main thing and exempts the subordinate both from the benediction preceding and from the one after it. It's not necessary that the subordinate thing be mixed with the main: rather, even if each is separate.
Even bread, which is most important of all, if it is subordinate — for example, say he eats salty fish and eats bread with it so it not harm him in his throat, he says a benediction over the fish and exempts the bread, as it is subordinate.
That which we say a benediction on the main thing, exempting the subordinate, is when he's eating them together or eating the main thing first. If, however, he's eating the subordinate thing first,... he says a benediction on the food first....
See there, and the commentaries, for more. In particular, there's a bit of a brouhaha about what benediction to make on a subordinate food eaten before the main food. As always, CYLOR for practical guidance.
Now, some have the following practice, cited with approval by Mishna B'rura (173:4): If they're going to be eating fish followed by meat, they eat something in between to clean out their mouths. (I've even heard some have a preference for bread for this purpose; perhaps there's support from the Rosh cited in Tur Y.D. 116.)
Would the above rule from 212:1 apply to 173:4 bread — bread which is eaten after fish and before meat for the purpose of cleaning out the mouth? That is, is such bread considered subordinate? (I'd guess so, since it's being eaten for a reason similar to the salty-fish bread's.) If so, then is it considered subordinate to that which is eaten before it or to what comes after?