I love to scratch build .. in part because I always seem to want to modify a structure to fit a location and scratch building fits nicely with that and in part that early on I got into cardstock modeling which again fits into modifying as needed.
I am grouping all of the railroad structures here. There may be some overlap with commercial or industrial but .. hey .. stuff happens.
Generally speaking this would be businesses such as a grocery, a barber shop, a machine shop - places that would 'sell' to the public. That machine shop though could also be industrial.
This would contain structures representing a manufacturing presence.There is an overlap between this and commercial buildings to some extent but generally these would not have a public sales presence such that a store would have.
This is where I will stick all of those things that support our infrastructure .. (hence the name) such as sewer gratings, pole transformers etc.