User Context is a feature of FAST Search for SharePoint 2010 that is analagous to using audiences with search results. As you start to test this out you may find that within the site collection you can create FAST keywords and document promotions and demotions, but when you try and manage the User Contexts for the site collection, you get an access denied. This can happen if your site collection administrator, even if you're a farm administrator. So what's the problem? Well in order to manage user contexts, you need to have some manage rights in the User Profile service application as well. The thing that can trip you up is that if you are a farm administrator you do have rights to manage the user profile service, but at least for now, that is not enough. You need to specifically go into Central Admin...Application Management...Manage Service Applications. Select your user profile service application and click the Administrators button in the ribbon. Add your account that you want to be able to manage user contexts, and grant them at least the Manage Profiles right. Once you save you changes there you should be able to access the manage user contexts page in a site collection.