Помогите, пожалуйста, понять, в каком значении здесь употреблено слово "sub-heading":

self-propelled lawn mowers, with a seat of sub-heading
- tractors of subheading, whose main function is that of a lawn mower
- four stroke mowers with motor of a cylinder capacity of not less than 300cc of subheading or
- snowploughs and snow blowers of subheading

ЗС!