Flat forks will usually have less wheel offset, so the wheel is more in line with the fork than with street & dirt forks which will have rake.
Dirt forks typically are a little lighter than street forks and will definitely not have 990 mounts on them.
Street forks will be built toughest of all and will usually have the option for 990 mounts as many street guys actually use their brakes and work the front wheel out there.