there was a big step up in how archiCAD handled 'materials' in AC17: as a result 'materials' became what things are made of and 'surfaces' what they were finished with (or how they appear).
the old 'structure' settings consisted a fill and it's pen settings and was largely a representation thing. from AC17 onwards you choose the 'material' that things are made of and the settings you have applied in your materials attributes give physical and representation properties to the element they are made of...
read about 'materials' or play around with the 'materials' attributes tab.
(geometry and positioning is the place to define an element's 'material'.
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