Exclusive Campaign: These will compete on priority basis. So if there were 2 CPM campaigns and 1 CPC campaign set all with a priority of 3 then impressions would attempt to distribute evenly between all campaigns.
Standard Campaign: With CPC campaigns when it comes to competing alongside CPM campaigns, it takes the lifetime impression figures divided by the amount of clicks and converts this to a CPM. So if your campaign is setup at $0.40 per click and your campaign had disbursed 10k impressions the prior hour, with 1 click registered, then the CPM attributed in the AdServer would be $0.04. If you have a floor price and the CPM is then below this the campaign will stop completely.
You can get around this and force the converted CPM to not alter and stay at a fixed price, you can do this by applying an Artificial Bid Rate (ABR). This will allow your CPC campaign to compete regardless of how many clicks have been recorded against this. Beneficial if you have a low priced CPC campaign.
You can also define a minimum CTR you would like your campaign to achieve however if the eCPM falls below the threshold click-through-rate then the campaign will be stopped. This is best used when you are representing multiple sites or if your site has a large taxonomy, this informs where the CPC campaign is achieving the desired CTR, subsequently can drive more impressions to those ad units.