ReadonlydirectionThe direction the selection's own shape implies, and the dialog's initial value.
The preset ids that can be offered for direction, in registry order.
A projection of what options would fill, or null where there is nothing to fill.
ReadonlyrangeThe selection the fill runs over — the dialog shows its address.
The addresses a fill in direction would read and write, or null where there is nothing to
fill — which is also how a surface knows a direction is not on offer at all.
Everything the Series dialog needs to offer, preview and describe a fill, without reaching into the model itself.
Remarks
The dialog is presentation: which types apply, what a setting would produce, and where the fill would land are all questions about the sheet, and they are answered here so the dialog does not have to iterate cells to ask them.