Point or also cell data acceptable for compute quartiles filter?

Or in other words: What is the meaning of FIELD_ASSOCIATION_POINTS_THEN_CELLS?

I have some data in the form of an “unstructured grid”, with data arrays associated with the cells. Now I want to display a “box plot”, but I am not able to directly proceed with the following pipeline as expected:

u-grid -> compute quartiles -> box plot view

Instead I need an additional filter to convert cell into point data:

u-grid -> cell centers -> compute quartiles -> box plot view

Actually the “compute quartiles” filter takes vtkDataObject input, but from the above I conclude that it actually takes only the point data from these data objects. So far so good, but then I looked into the source code of the “compute quartiles” filter and there I see that actually it is more complex.

Namely the data that it tries to obtain are described as “FIELD_ASSOCIATION_POINTS_THEN_CELLS” (and not e.g. FIELD_ASSOCIATION_POINTS which is also available). Intuitively I would understand this as either of the two following:

a) “take point data, and then append also the cell data”
b) “try to take point data, and if not available take cell data”

But both do not seem to be the case, because if it was a) I should see my cell data directly (without the “cell centers” filter), and also in the case of b) I should see the cell data because there are absolutely no point data available - except the point ids and point coordinates.

In other words: Empirically I am getting the message “only point data are processed for box plots”, but from the source code I would expect that cell data SHOULD also be possible.

So is there anybody able to explain this behaviour in a few words?

(Sorry for complaining, but the title checker automatism is a real pain…)