To the civ's that have these functions enabled, a post or pre-conquest detail on what should happen to the inhabitants after capitulating a settlement would add depth and a bit more purpose to having these tags in play.
Dwarves for instance can only arrive to raze the settlement or take it in the current scheme of conquest controls.
Enslaving - All the remaining inhabitants now have the profession of 'slave', this commodizes them when they emigrate or are moved to belong to masters who desire one (acceptable vs personal matter ethically as a open choice moral question for citizens), and the site will be more productive with tribute being worked by the slaves constantly,
prisoners, POW's and criminals can be transferred here carried in a cage to map edge but innocent civilians can't and it won't contribute to total 'hill dwarf' populations.
- For these reasons, areas that are unliked by your race, or unsuitable building type for habitation make excellent hubs for slaves, slaves can also be took in raids via a seperate option (see below for more detail on slave role & AI)
Slaves moved to your fortress cannot be assigned to normal labours and fufill a similar role to semi-intelligents in hauling, assignment to certain civilians to otherwise do jobs for them and working in a military squad against their will, the assigned slave will be listed within the person's posessions and in adventure mode if they are seperated may offer a quest to return them.
- Certain nobles like particularly villanous lords may 'require' slaves amongst their desired property like a large room, or a armor rack configurable within raws and these will in the noble's stead handle any manual labour required, fetching food drink and objects allowing them more spare time to lounge around in social areas and fufill needs the slave can't
Mutilating - Remaining inhabitants will be massacred and the site entirely cleared for habitation for the conquerer with all the 'cheery' punishments left behind as usual, a forseeable problem with the hill dwarf's conquering arc of having simply too many people to micro manage in your civilization without inviting them to your fortress and suitably robbing & dumping them in lava.
Just some hopefully reachable and reasonable short term suggestions while Toady's still hovering around the topic of what to do with prisoners and the conquest mechanics which are very light.