Choose which axis points up.
One complication with many of the common 3D mesh formats is that they do not store which direction is "up" in the model. Unfortunately different 3D programs are roughly 50-50 split between "Y-up" and "Z-up". If you are using 3D printing software, you are mostly dealing with Z-up, while if you are using film & games software, or Autodesk Fusion 360, chances are you have files that are Y-up. This is a nightmare!
Inside Meshmixer, we use Y-up. This decision was made years ago and changing it now would be a huge amount of work. However, since many files are Z-up, we provide an option to automatically handle the Z/Y-up conversion on import and export. This option is available in two places: in the dialog under the File tab, and also in the Append/Replace dialog. The setting is called Flip Z-Y axis on Import-Export, as shown below.
