The mainstream includes different offers from sweepstakes, mainstream dating, health and beauty, download and other verticals. Mainstream vertical accepts all traffic types and covers all geos.
Paid actions are different and depends on offer, it could be soi/doi registration, any subscription, sale, etc.
The most suitable traffic for this vertical is common mainstream, e.g. if you target traffic only for sweepstakes or you have health and beauty audience it's better to run direct offers that you can pick via our CPA marketplace.