Table 300.5 is confusing. The most important item is row 1 column1 -- direct burial cable for all uses except the rows under it is 24". All of those other rows cover specific areas or coverings (buildings or concrete). I think the intent here is further protection and not less. Look at column 2 and you'll see RMC which is normally buried 6" has to be increased to 18" or 24" in most of those other rows. But if you're running this under a dwelling only driveway or parking area (perhaps even a gravel one), then you may be able to only go 18".
