If I use the modifier Solidify to define the thickness of an object, do I use negative or positive value? For example 0.1 or -
-0.1? (it is a scanned object)

A further question:
If I apply the solidify modifier how can I select the inner resp. outer shell of the object in the edit mode?

Thank you very much, KIMA2013

Default cube, delete one face and see how it works. Whichever you need for your object.
One can assign different materials for outside, inside and rings before Solidify is applied. That helps to select vertices By Material later.

Depends on which way your normals are facing. Positive value in the modifier is the opposite side of the normal direction. If the normals aren’t consistent, it makes a mess of it.

Afaik, you can’t easily. Modifiers should be left in the stack and only apply if absolutely necessary.
If it’s a closed mesh, you can select one or more vertices from either part and ctrl+L to select linked. If it’s not, then perhaps hiding vertices from the edges so that the separation is clear and then select linked.

Edit: thanks eppo. Forgot about the material offset. That assigns different materials and after applying, one can simply select one part and select similar materials with shift+G.

I always wondered what the Material Index Offset and the Rim setting next to it were exactly for in the Solidify modifier, and after reading your reply and testing i finally understood.
Just create 2 different material of different colors, assign the 1st one to the existing model, then change the material offset (and rim) from 0 to 1 and they recieve the 2nd material

After applying it’s then easy in the Material panel to click on Select to select the faces with the material assigned.

Thanks a lot for this, i was always annoyed when i had to reselect manually the outer or inner part of what a Solidify had done (especially as when i tried to use vertex groups i noticed the Solidify annoyingly copy the vertice of the generate part into the existing vertex group without asking you).

That material difference works great !