This page explains the way buyable items in the game are added to stores on planets.
Tech definitions are located in dat/tech.xml. The syntax is described in the article on Tech.xml. This file defines tech groups, each of which contains a list of zero or more items. An item can be either the ID string of an outfit from dat/outfit.xml, the ID string of a ship from dat/ship.xml, or the ID string of another tech group. If a tech group is included in another tech group, all its elements are added to that tech group. Be careful not to recursively add tech groups to each other!
Tech groups can be assigned to planets by adding them to the <tech> node in dat/asset.xml. Individual ships and outfits may also be added directly here if desired.
Techs and the unidiff
The unidiff has the ability to add one tech to another. This makes it possible to add new items to outfitters or shipyards as the game progresses. Here is an example usage:
<unidiff name="heavy_weapons_license"> <tech name="Basic Outfits 1"> <add>__Heavy Weapon License</add> </tech> <tech name="Basic Outfits 2"> <add>__Heavy Weapon License</add> </tech> </unidiff>