Yes, each question does have a character limit.
Windsor only has 2 categories: general and indigenous/Aboriginal. When applying as a mature or access student you should be explaining your circumstances in that optional question.
This just happened to me in firm with a niche practice area (municipal/land development/condominium). I just checked the going rates for the firms in the area using the ZSA chart, and some of the salary spreadsheets on this site to figure out a baseline. But, it might be worth figuring out what your area of law tends to pay in general if the practice area is really niche or in high demand. For example, I ended up getting about 30% more than the going rate because the municipal sector is really hot right now, and I wasn't a superstar candidate.