PX leads can be Exclusive (sold to only one advertiser) or Shared (sold to multiple advertisers at once) to optimize the ROI on your lead campaigns.
Usually Exclusive Leads are more expensive because only one advertiser receives the Lead with contact information of the customer. However, sometimes Shared Leads are sold to several advertiser and can be more profitable as a result.
For example, one lead can be exclusively sold for $50, while shared price for it is $15. Considering that Shared Lead can be sold to 5 different advertisers, total Shared price will be $75. This gives you as the client of PX a high degree of flexibility to maximize the revenue on your leads.
Choosing the sales model highly depends on vertical, platform settings and preferences of each particular advertiser. Further, there are even some less common but highly powerful models like Multi-leg and Undersold models.
Read more about all of the sales models that PX supports.