This video is about implementing a custom validation provider for XAF, or if you believe Gary (this is still Oliver writing here), it's about GIGO - Garbage In, Garbage Out. In any case, XAF provides the means to keep users from entering more garbage than is good for them, and in many cases this means extending the validation system to support checking for conditions that depend on your business logic.
It should be said that this video was created on the basis of XAF 8.1, and there are some changes in the validation code for 8.2, so if you're trying to copy the sample code from screen, your mileage will vary. I'll create an updated sample for 8.2 and post it later - my apologies for this inconvenience. Anyway, the information is the same and it's still fun to watch!
Click the image or this link to watch the video!