It's what makes a PAC different than a Super PAC. The expenditures are independent of the campaigns of the candidates they support and its all the Super PAC can spend. Super PACs are more accurately called "independent expenditure-only committees." But the expenditures are limitless, which explains their popularity. The problems is determining if the expenditures are in fact independent - if they are not coordinated.
This link in Ballotpedia helps explain how this is done: Click here for detail.
