I'm using for that purpose: flags duplicate Or is there a reason not using this for that?

"flags duplicate" means "Hi Therion, this is an underground leg. It goes down a passage that we have already surveyed. Please do not add it to the surveyed length." Duplicate legs therefore do not add to the cave length. But they definitely do contribute to the cave depth (if they happen to be the highest or lowest point). And when you generate a .lox file or .3d file, they will be treated as a cave passage, not a surface leg. Therion will draw walls around them, and use them to determine the size of a cave passage. Survex Aven will show them as an underground (but dashed) leg, and will therefore make it look like the cave goes that way. Similarly, "flags splay" for named stations will also not add to the cave length. Therion will also not consider them to be part of the cave depth (even though they could be the only way you measured up the tallest aven, unfortunately). But they will tell Therion how to build the cave walls with .lox files. They also get treated differently when outputting extended elevations. "flags surface" is the right thing to use for surface. You can even use "flags surface duplicate" for surface legs that are duplicated, which has no practical effect (except I think it might give different calculations when you export a survey list).