Pen -1 (& pen 0) are special pens for the background colour of fills.
They can only be used for fills and report an error if selected for any other purpose.
The problem is that any pen parameter in a GDL object will have the option of pen 0 & -1- they just can't be used as it will give an error message.
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As mentioned here .... https://archicad-talk.graphisoft.com/viewtopic.php?p=314295#p314295
What if I need these values to be saved in an array ???
I would expect that your answer is "Then use an Integer Array Parameter", but based on the circumstances I have mentioned before, including those users that aren't versatile in GDL scripting, and maintaining library part's scheme & structure with Graphisoft's GDL styling, it breaks the object's programming scheme, luckily it only affected one portion, but if it was something else it would have caused a big mess
A good friend of mine have once told me that I´m so brute that I´m capable of creating a GDL script capable of creating GDLs.