in Emu Loader you can simply ignore the <slots> because they have negligible effect on MAME
More complete answer:
The slot options come from MESS slot device code and they're used to support configurable slots in computer emulation where e.g. you can have 8 ISA ports to connect the combination you want of Floppy drives, Hard Disks drives, Serial mice, Graphics cards, Sound cards and so on (and yeah, current MESS basically allows you to test any combo you wants in PC and Mac drivers) or you can plug a IEEE48 serial cart into a Vic20 and connect several disk drives to the system through it (and yeah, current MESS allows you to do this and much more both in Vic20 and C64 drivers)
In the case of MAME, slots cannot be configured by users and only a default configuration is offered (which should match the configuration of the hardware inside the cabinet)
Hence, expect to see more of these when emulation of PC-based arcade systems become more accurate, but don't expect the users to be able to play with them as they can do in MESS
Focusing on your specific example, taitogn.c games, the change is related to the presence of an "IDE Controller" in the original cabinet and in the changes of its emulation performed by Micko between 0.145 and 0.145u1. Now the IDE controller is handled through slot devices and its emulation is hardcoded to have an HDD connected to each of its slots (this is in progress, the final configuration will reflect the real content of the cabinet, i.e. only a pcmcia card connected), hence the xml output.
Killer Instinct is another example you can find in xml.
Feel free to ask, if you need any additional info.