ARES Commander has no 2D FLATTEN command, but it is already in our wish list/been submitted as an Enhancement Request. As workaround attached a LISP routine that can be used to move 2D entities to Z=0.0, if they are located in the world coordinate system. You have to save the LISP file somewhere, then you run LOADAPPLICATION command and select the file CHANGEZ2ZERO.LSP. To execute the command type CHANGEZ2ZERO.
If you first change the view direction to a side view, you can select only the entities in a different level.
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For 3D Solids you have to use the command _MAKEFLATSNAPSHOT from the 3D Modeling workspace under -->Solids and Meshes-->Make Snap Shot.