Design includes the base elements upon which components and pages themselves are built. The design category gives guidelines for creating pages that truly represent the IxDF UI Kit philosophy. That is why it contains typography details and guidelines to use it, similar information is provided for elements like elevation and so on.
The elements in this category are used in every single page and are part of components themselves. So any change here will fundamentally change the feel of the website. So great care should be taken while making additions or changes to these elements.