Save as AC17 version in AC18.
Then open it in AC17 and save it as AC16. This is the only way.
However, GDL Objects will probably be missing in the backsaved file. Also, any element/data created with Tools/commands specific to AC17 and AC18 will not be there in AC16 since in that version they didn't exist.
What is not called a Surface was called Material in AC16. I suppose your AC18 Surfaces will save back to AC16 as Materials.
Loving Archicad since 1995 - Find Archicad Tips at x.com/laszlonagy
AMD Ryzen9 5900X CPU, 64 GB RAM 3600 MHz, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), Windows 10 PRO ENG, Ac20-Ac27
