What does the "[u.]" next to the tag value mean in the WinCC value table control?

After having positioned the ruler on the trend view, sometimes the values displayed in the Y value column of the value table are followed by "[u.]" or "[i.]"; what do these mean?

Late answer, but from the fine help ("WinCC Information System --> Working With WinCC --> Archiving Process Values --> Output of Process Values -->Process Value Output in Process Pictures --> Process Value Output in the Form of Trends in Process Pictures --> Operation in Runtime --> How to Determine the Coordinates of a Point"):

The displayed values can be assigned an additional attribute in the form of a letter:

Letter "i." : The displayed value is an interpolated value.

Letter "u." : The displayed value has an uncertain status. The value is not certain if the initial value is not known after runtime has been activated, or when a substitute value is used.

//PerD