Xactly, a San Jose seller of an on-demand sales compensation management product, said it has raised $15 million in a third round of financing led by Alloy Ventures.

Existing investors Bay Partners, Rembrandt Ventures, Outlook Ventures participated, as did Spinner Asset Management.

Here is the statement, which lists the company’s customers and more about how it works.

In correspondence with Xactly’s chief executive Christopher Cabrera, we asked how Xactly’s software differs from competitors such as Callidus and Centive.

Cabrera was senior vice president of operations at Callidus, until leaving to create Xactly. Here’s his response:

In my account calling at Callidus, I was repeatedly forced to walk away from sales opportunities because it was clear that only the large, Global 2000 organizations could afford the large up-front software license and maintenance fees, hardware, implementations and complex upgrades of an enterprise application. Mid-market sized companies simply could not afford the cost of an enterprise incentive compensation management solution. They needed it. They wanted it. But it was beyond their financial reach. I saw this unserved mid-market opportunity, left Callidus Software and went on to found Xactly Corporation on March 1, 2005.

He said Xactly delivers a purer “on-demand” product that either Callidus or Centive.