The inner post arrangement is possible to do in a different way as well, as you can see on my attached screenshot:
There are a few points that are worth making about how this can be done:
1. The most important one is that it is possible to have more than one Inner Post within a single Railing Segment as you can see on the above screenshot. You switch to the "Inner Post" page of the Dialog, and with the use of the "+" button in the top right corner of the preview area, you can add multiple Inner Posts. Archicad will always take the rightmost of the placed Inner Posts and consider that to be the right edge of the Segment.
2. When specifying the position of the individual Inner Posts, it is possible to lock their distance to a specified value from the left edge of the Segment/Inner Post to their left by clicking on the LOCK icon to the right of the "Distance to Previous" field. In this example, we have 3 Inner Posts. If you lock the "Distance to previous" value for the first and third and set them to the same value, you can achieve the symmetrical look shown on Red's image.
3. You can also define the top and bottom of Inner Posts relative to Rail 1 and Rail 2, which are the upper and lower boundaries of the first and second Inner Posts on Red's image.
4. Regarding the Panel, I, too, would probably create a custom GDL panel, that consists of that grid or net, with its thickness, width and height, plus X and Y offsets for starting position of the grid as parameters. Or, Barry's image panel suggestion would also work.
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