Appraising it for business income is a going concern and that would make it a commercial appraisal, which a res license typically does not do -
Regarding HBU of a property, I like to imagine it as vacant - that way, one does not get sidetracked about who actually lives there or rents there and what activities they engage in. A house and a 4 stall barn is probably a hobby farm. A property with a house with a 20-stall barn and extensive facilities is typically set up for boarding. Especially if the RE agent is bragging about it. Seems the client asked the appraiser how many stalls there are and if there is any income present. Which should have been disclosed anyway.
WRT HBU, the question for a residential loan is whether to continue the present existing use or other . If a horse farm or hobby farm is on a large lot or a parcel that can be subdivided, in some areas, especially semi-rural, where development is encroaching, the land can be worth a lot more than the improvement. HBU is other/ land value in that case. I grew up in an area that used to have many stables, most of which have closed and been made into subdivisions. Kind of sad, but a clear illustration of HBU. In a rural or wealthy equestrian area, it is more likely that the horse farms will remain.
There is a saying- how do you make a small fortune with horses ? Answer: start off with a large fortune.