ProjectsInCategory will return a list of Project ID's for each project belonging to the supplied project category.
The function can take a list of one or more project categories, identified by either name or unique Identifier.
ProjectsInCategory(categoryNameOrId, categoryNameOrId, ...)
Many customers use project categories to seperate sandbox/sample projects from production projects, if the category for test projects is "Sandbox" then you can exclude all those projects requirements using the following query.
EntityType = Requirement AND Project NOT IN ProjectsInCategory('Sandbox')
An example of getting projects belonging to two categories, one identified by name and the other identified by it's unique Id (in this case an 'iPhone apps' project category with unique ID '743A4D99-0B4A-4356-BC95-057C20D00ADF').
Project IN ProjectsInCategory('Android apps', '743A4D99-0B4A-4356-BC95-057C20D00ADF')