How to understand the meaning of water mask 256 * 256

[libofei2004]

The description of water mask in heightmap-1.0 terrain format is as follows: "The child bit mask is followed by the water mask. The water mask is either 1 byte, in the case that the tile is all land or all water, or it is 256 * 256 * 1 = 65536 bytes if the tile has a mix of land and water. Each mask value is 0 for land and 255 for water. Values between 0 and 255 are allowed as well (but not currently present in the data) in order to support anti-aliasing of the coastline." How to understand " it is 256 * 256 * 1 = 65536 bytes", The tiles of heightmap are 65x65 vertices and overlap their neighbors at their edges. how can i set 256 * 256 water bytes in a 65x65 tile?