I love this tip for improving the definition of your products. If you want a really clear idea of what your product is, articulate what it is not with an "Is not" list.
The "Is Not" list does not exist to spell out a bunch of irrelevant and silly product features that would be unlikely to find their way into your product. Instead, it is confined to the close call features that might exist in future products and that cause a great deal of debate. The "Is Not" list quickly also focuses on the most difficult product definition decision, which is not what to do but what not to do.
I have seen this used to tremendous success in more design-oriented product briefs to really help provide a vivid picture of a product before it's created. It can narrow very clearly the range within which a team further defines and executes the product.
An Is / Is Not list....
- Does provide contrasts but does not list opposites.
- Is about making features apparent, but not about listing apparent features.
I think you get the picture!
Source: Results Without Authority: Controlling a Project When the Team Doesn't Report to You -- A Project Manager's Guide