Ann: Quarterly Activities Report and Appendix 5B - Sept 2018, page-10

To my understanding, it means they have acquired parcels of land in 21 different 640 acre lots. ie: Think of each 640 acre section as a block - They own a piece of each of the 21 blocks. The higher percentage they own in each block, the more likely they are able to acquire the remainder of the block. If another competitor owns a chunk of the block, BRK may choose to trade their piece of the block / swap for pieces the competitor may own in other sections and vice versa / sell it to the competitor or choose to buy the competitor out to acquire the full block.

Happy to be corrected but this is what I understand of the model and the way the leasing side of things works.