A scoped UDA is a UDA that has an artificial scope defined by the user. For instance, a UDA called Status has no scope whereas a UDA called Project:Status has a scope. These UDAs are just a way for you to keep track of what the UDA applies to; Status applies to all data types while Project:Status should only apply to the project data type (although there is nothing stopping you from referencing a project UDA from the metadata tab in your project preferences).