OpenTTD maintains a minimum number of industries in the world based on map size, year, and the industry density selected by the player. This method is an obstacle to NewGRFs controlling industry availability, for example to prevent steel mills before 1856, because the game doesn't reserve slots for industries which aren't available yet. Once available, the new industries only appear when another industry happens to close.

It would be simple for an industry NewGRF to calculate and reserve slots itself, but NewGRFs cannot detect the industry density selected by the player to calculate the intended number of industries in the world. If the selected industry density were made a global variable, it would give NewGRFs all the information needed to augment base game industry generation logic while seamlessly respecting the player's chosen settings.

• List of global variables
• Global variables in NML
• Test NewGRF