UX Core & UI Core
An important milestone of the project is the development of the "project soul." It accounts for 30-40% of each project designing, and in terms of the available functionality, it offers the main path of the user, i.e., the process through which the user can go to meet their needs. Project soul consists not only of the main assumptions of usability matter but also the visual style because its screens also include the graphic design of the UI.
Project soul is a part of the project ready for usability testing. It is a starting point for the validation of the concept because on its basis you can determine how the product will look like and how it will meet the essential user's needs. It is on its base that a product prototype is created.
Development of the project soul also usually allows you to speed up the rest of the project work because the design of the remaining content is based on its assumptions. Also, the project soul will enable you to present to stakeholders how the core of the product will work and look at the end of the entire project. Very often it covers most of what, after programming and implementation, is referred to as the minimum viable product.