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it comes from the descriptions in the classes.
here is Bonus Feats class Feature from the fighter the bold part is mine for emphasis.
....a fighter gains a bonus feat in addition to those gained from normal advancement (meaning that the fighter gains a feat at every level). These bonus feats must be selected from those listed as combat feats, sometimes also called "fighter bonus feats."
this means that from the fighter you can pick any feat as long as you meet the prerequisites and has the word combat after its name.here is the brawler's Bonus combat feat class Feature
...a brawler gains a bonus combat feat in addition to those gained from normal advancement. These bonus feats must be ones that affect or improve her defenses or melee attacks. The brawler must meet the prerequisites of the selected bonus combat feat.
this means that the feat from the brawler has to be a combat feat that can't improve things like ranged weapons or a Combat Maneuver.