OK, the first two...
"...the material assigned to a body from "tools -> materials -> assign materials"
and the materials assigned during the simulation..."
...are one and the same. If you assign a material using "tools -> Materials" and you were to later enter Simulation, it will show you that the material has already been assigned. Now you can override it there if you wish, but if it's already what you want, you don't have to do anything else.
The third item, materials assigned when rendering a model, these are for 'visualization' purposes ONLY. It will have no effect whatsoever on the density or what the simulation package is using.
As for the fourth item, 'Solid Density', this was a precursor to the ability to assign an actual material using the first/second item above, and it ONLY does that, assign a density to a solid used when computing Mass Properties. Note that if you assign a 'Solid Density' and then assign a Material, the Material assignment will overwrite the first density assignment.
I hope that's clear as to what these are used for and their relationships to each other, when there is any.
John R. Baker, P.E.
Product 'Evangelist'
Product Engineering Software
Siemens PLM Software Inc.
Industry Sector
Cypress, CA
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