Just to clarify my understanding of what it is that makes them different to other lines;
1. by default have –outline out set, and so define the boundaries of a scrap
Yes.
2. by default have -clip on set, and so some symbols are clipped at the wall
No. By default walls have "-clip off", which means they are not clipped by scrap outline. If any line has "-outline" set to "out" or "in", it has automatically -clip set to "off".
3. have their own special subtypes, and altitude labelling, that are not available to other lines
Yes.
4. are used by the scrap join algorithm to decide which lines to join
Yes.
However, is it really the wall line-type that the
join scrap1 scrap2 7
statement uses to decide how to join the 7 passages that connect scrap 1 with scrap 2.
Or is it the –outline in/out property of the lines that the join statement reacts to? (as could almost implied from outline definition pg 28 of Therion Book)
Therion seeks scrap ends at ends of visible wall lines. No other line types are taken into consideration. Best regards, S.