U - stands for User!
UX. The user's "experience" of moving through the site. Emotions felt. The first thing to consider. Wireframing is useful here, tools such as Balsamiq, Lucidchart or Mockplus.
UI. The actual interface - look and feel. Sketch and Figma are common choices. For implementation, Bootstrap / Material UI provide ready-made themes to choose from.
User Name. Are we talking about logged in user? system user? User ID or Name? For consistency of name across SAS9 and Viya we made a dedicated macro.
User Guide. Apps should generally be intuitive to use, however a User Guide (and Admin Guide and Developer Guide) should be delivered with every project. Tools for this include MkDocs (site generator) and good old MS Word!
User Engagement. Users should be exposed to the app as early as possible in the development cycle (Sprint 1 onwards) to ensure the app is fit for purpose.