Thanks a lot Michael.

You understood perfectly well what I meant, I'm french speaking, so I make some mistakes in English.

And yes it was about the "weld" option, not merge. Inside one 3d package it can be confusing, weld, merge, join, collapse, etc ... so between different software it's even harder to understand sometimes, it can be confusing.

For the open edges, it can be useful to let them open during export sometimes, so it's good to have an option to weld or not, joined edges in this case.

For the "joined surfaces" vs "solids" I will try to build a simple shape with different methods, to figure out how it works, when the rebuild command make sense for ex, to have clean geometries that are commands friendly.