The example in the documentation shows that it is a table of independent and dependent values, but does not specify the actual type of said object.
The type of the relevant variable can be easily found in the 'Plot Traces and Attributes'-window.
(It is the window that pops up when you want to draw a new plot in the data display)
Simply click on the variable that you would like to access in the left-most column and click on the 'Variable info...'-button.
This will give the dependency, Num point, Matrix size and type of the variable.
When defining a new variable using an Equation you can also easily find the type by changing the dataset selection 'Datasets and Equations' (in the 'Plot Traces and Attributes'-window) to the 'Equations'-tab. Your newly defined variable will be there and the same procedure applies.
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