Using an example to illustrate: if you live at "350 West Street", your street number would be 350 and West Street would be the street name. Building number, house number, and street number all usually refer to that number that comes before the street name. Some places don't use street numbers and just name their buildings, in which case some people choose to put "0" or "N/A" in the street number part. My boyfriend and I both thought it was really weird to put the street number separate from the street name and to make it mandatory. We've never seen anyone do it that way before.