From Elite Wiki

shipdata-overrides.plist is added with oolite 1.72.

It loads after all the shipdata.plist data are read in. It contains the same keys as the normal shipdata.plist. Whenever it finds a match with a key that also exists in shipdata.plist of any oxp, it overwrites the entries defined in shipdata-overrides.plist but leaves the other as they were. e.g.

"adder-player" = {
 max_energy = 95;
 thrust = 25;

Will change only the max_energy and the thrust of the adder-player ships. All other data remain unchanged.

See OXP_howto for a overview about the other plists.