Dxf files have a version. Depending on this version, some features may be available, but not others.
Otimize Nesting currently supports DXF version 2000 or later.
To avoid compatibility issues, there is a practice among professionals to use very old versions of .DXF.
The reason for that is to be on the safe side and ensure files are opened on another computer, with other CAD software installed.
However, this procedure has a cost. Loosing important resources.
In most CAD programs you can specify at the time of saving the file which format you want to save.
These programs also have the functionality to tell which .DXF version is used when the file is opened, as shown in the following image:
The image, from the QCAD software, shows that the .DXF file was created in R15 or version 2000 format.
Below is a table with the versions and codes published by the .DXF format
References: