I was going to ask the question, but figured it out partway: to set the core structure of your column, right-click on the icon in the centre of the Column Settings panel, then choose Structure of ARCHICAD Column.
I've also noted a strange behaviour with the core dimensions. Core Dimension 1 is in mm, but Core Dimension 2 is in metres...unless it's a round structure, then it's in mm. Go figure.
This is in ArchiCAD 21.
