If Coverage A provides insurance coverage for the dwelling and things that are attached to the dwelling, "Coverage B Other Structures" provides insurance coverage for all structures that aren't connected to the dwelling. In this case "structures" is a complicated way of saying anything that is constructed. For example, a detached garage (remember an attached garage would be coverage A), fences, storage buildings like sheds and greenhouses, in ground pools, barns, sports courts, gazebos, walkways, and driveways.
Usually the limit of coverage A will be defaulted to 10% of the value of the home, although more can be purchased. This limit is a "blanket" meaning you will have one limit for all your other structures, be sure you are purchasing enough to cover your needs. For example if you have a nice 2 car garage, pool, or mother-in-law quarters you probably don't have enough coverage.
Coverage B Other Structures will have the same coverage form and loss settlement as Coverage A Dwelling. Again, the special form is always the best form. Replacement cost is always better, but you need to get the proper limits for it to work best. Also, it is important to note that if you intend to rent out an other structure such as a pool house, mother-in-law quarters or something similar or use it for business, coverage can change. If you have a situation like this explain this to your agent to ensure you get proper coverage.