I have been working with bindings and selections and while fiddling with an example I had in mind I noticed something peculiar in the docs which I don't understand - the usage of fields in selections.

Take this example from docs -
Important code -

slider = alt.binding_range(min=0, max=100, step=1, name='cutoff:')
selector = alt.selection_single(name="SelectorName", fields=['cutoff'], bind=slider, init={'cutoff': 50})

We will focus on name in slider and fields in selector.
Q) I think name is just a string placeholder used for displaying purposes?

Based on what I read here, fields are nothing but dataframe columns and encodings are self-explanatory.

I looked at the doc for selection_single and corresponding doc from vega-lite. But it still doesn't make sense to me.

Why?
There is NO cutoff field in the dataframe we pass to Chart.

Moreover, if you run the code below, where we replace cutoff with xval which is a field in the dataframe, then we see the exact same output.

slider = alt.binding_range(min=0, max=100, step=1)
selector = alt.selection_single(fields=['xval'], bind=slider, init={'xval': 50})

alt.Chart(df).mark_point().encode(
    x='xval',
    y='yval',
    color=alt.condition(
        alt.datum.xval < selector.xval,
        alt.value('red'), alt.value('blue')
    )
).add_selection(
    selector
)

Can you please tell me what is the role of cutoff (I know its purported function) and fields here?