You can use the 'default' context condition to ensure that at least on context within the selected namespace an attribute is active on any page of you site.

Context will first evaluate all other context conditions, then of none of those have already matched it will try to active a default context within each namespace/attribute pair if one is present.

Example

Let's suppose that you have a block that you want to show on every page of your site, except in the 'news' section. Here's how you could do that:

1. Create your first, sitewide, context, set it using the default context condition and select the block that you would like to show using the reactions section.
2. Make a note of the value of this context with the chosen namespace and attribute.
3. Create your second context, use exactly the same namespace and attribute as the first.
4. Set up your second context with the conditions that define your 'news' section.
5. Set up the reactions to be the same as your first context, but without the block that you don't want to appear.

That's it! Context should evaluate your contexts and if the 'news' context's conditions are met the blocks it defines for that namespace and attribute will be used otherwise the defaults will.