Revit uses the Project Browser to organise ‘Views’ in an organised manner. Views can be listed by Discipline, Sub Discipline, Scale, etc. In fact almost any property can be used to organise views.
However, if the views are not fully defined Revit will not know how to organise the view and will be placed in a temporary location until the required parameters are provided. That temporary location is identified by ‘???’ in the browser.
As an example, the view shown in the image above knows that it’s Discipline is ‘Mechanical’, that it is a ‘Floor Plan’ but does not have a value for ‘Sub-Discipline’ therefore accordingly, the view is listed under the ‘Mechanical’ discipline heading under ‘Floor Plans’ but because no value for ‘Sub-Discipline’ has been supplied, three question mark symbols are used (???) in place of a ‘Sub-Discipline.
However, when the ‘Sub-Discipline’ value of ‘HVAC’ is defined, Revit knows to store the view under that heading (and removes the three question marks).
In summary, define the views ‘Sub-Discipline’ value to avoid Revit displaying ‘???’ as a heading for ‘Sub- Discipline’ within the browser.