What is the difference between Accural and Volume offer

Hello,

I'm struggling to find the difference between the Volume and Accrual offer?

Further, In the Channel Revenue management guide they have given an example which is exactly the scenario i have at my client side. I want to understand how this can be setup in Trade Management

For example, an organization creates an accrual offer for Wal-Mart for the period March 1, 2004 through March 31, 2004.

Wal-Mart gets $1.00 for every unit of Product X that Wal-Mart buys from the Manufacturer.

Upon completion of the time period, the accrued amount can either be paid back to Wal-Mart or can be accrued for use in future promotions.