What offers does Mainstream vertical include?
The Mainstream includes different lottery-offers, e.g. win an iPhone/Samsung/PS or different vouchers, etc.
Paid actions are different and depends on offer, it could be soi/doi registration or CC submit.
The most suitable traffic for this vertical is common mainstream. As for sources it can be FB target, push notifications and others.
This vertical will be available for you, only after approval. For approval please send the request and provide your manager with your traffic sources.