A Hex Number is a unique identifier for a location of a hex in the subsector, quadrant or sector maps.
The mapping convention for a sector is a grid of 32 x 40 hexes, a quadrant is 16 x 20 hexes, and a subsector is 8 x 10 hexes. Each hex is given a number consisting of the horizontal position (number from 1 to the number of hexes) followed by the vertical position (numbered from 1 to the number of hexes). These numbers are given a leading 0, so both positional numbers are two digits. The starting position is always in the upper left corner of the map.
For example, the hex number in the upper left corner is always "0101", with the hex immediately below it is "0102", and the hex to the right would be "0201". The hex number in the bottom right of a subsector would be "0810", in a quadrant would be "1620", and in a sector would be "3240".
Sector Mapping hex numbers
Because a sector consists of either 4 quadrants or 16 subsectors, it is frequently the case the hex numbers presented in the quadrants or subsectors of a given sector are numbered based upon their position within the sector.
For example, Subsector E would have the upper left corner hex as "0111", and the Subsector F would have the upper left corner hex as "0911".
When presenting these smaller map units as part of a large grouping, the sector hex numbering scheme is frequently applied.
By convention each hex in the astrographic map is approximately 1 Parsec across. Which, by the geometry of hexagons, means they are 1 parsec from center to center. This makes distance calculations simple (just count the hexes). The size of the hex matches the Jump Drive ratings, in order to make the calculation of possible destinations as simple as possible.
Depending on the scale of the map each hex may or may not have an explicit hex number present on the map. In some maps only the hexes containing system have an explicit number in them, in large scale maps every hex may have a number in it.