In short, both.
The Sulvo seller account reflects earnings in vCPM (Viewable Cost per Thousand Impressions), but that doesn't necessarily mean that all buyers were buying on a CPM or vCPM basis. CPC (Cost per Click) and CPA (Cost per Acquisition) bidders are converted into vCPM values to ensure you are getting paid for each ad impression.
A higher CTR (Click-Through-Rate) rate does correlate with higher CPM rates over time, but there are some brand advertisers that don't necessarily aim to buy clicks. Instead, for example, some might be interested in highly viewable placements and impressions to drive brand awareness or offline conversions.
Most buyers, however, are buying on a vCPM model (viewable CPM), which means that higher-viewability ad placements tend to vastly outperform low-viewability ad placements. This typically happens because:
Visitors are more likely to interact with the viewable ads
Buyers are paying a premium to ensure that their ads are more likely to be seen
Just so we are all speaking the same language:
CPM: Cost per Thousand Impressions
vCPM: Viewable cCost per Thousand Impressions
eCPM: Effective Cost per Thousand Impressions
CPC: Cost per Click
CPA: Cost per Acquisition (or cost per action)
CTR: Click Through Rate