What is Project Explorer 'Harvesting'?

Dear Forum,

ive come across Project Explorer recently and not having used it previously, it seems quite useful. Upon testing it, when i click 'Models' PE then harvests a lot of files. Ive not been able to get to the end of its harvest to see what its harvesting. Any ideas as it seems to be harvesting more than what is directly referenced, which is what i assumed is the harvest?