Well, one of the most important considerations is skin dryness. If the person is active or not, wetness against the skin allows bacteria to grow, the formation of ammonia, etc... This can lead to urinary tract infections and skin breakdown, not to mention odor. The "lower end" briefs, in my experience, are not good at skin dryness or odor control. In the case of fecal incontinence, skin cleanliness and immediate change of clothing/diaper/pullup, whatever is a MUST. Feces can be acidic and cause very quick skin breakdown and the bacteria in feces is an almost guaranteed urinary tract infection, ESPECIALLY in women.
If the person is not mobile at all, "bedridden", if you will, skin dryness is a MUST. Being immobile is hard on the skin anyway, without the added issues caused by wetness, bacteria formation, etc...
Obviously, since wetness protection is critical, you want to make sure the person is wearing a product (whether it be an external catheter, pad, diaper...whatever) that does not leak urine/feces on to their clothing. If diapers or pullups are being used, the fit is absolutely critical to preventing leaks and skin issues. Too tight fitting puts pressure on the skin AND will cause leaks. Too loose fitting, and you have leak issues.
If you have specific questions about caring, please feel free to post them. I'm sure those of us here can help out with a wealth of knowledge!
I hope this helps!
Number one consideration should be skin care. As mentioned above. You did not say what level of incontinence. Or what type? But any incontinence type or level. Taking care of the incontinent persons skin should be the number one priority.