What is DXF?

The common assumption is also the cite of Wikipedia:

AutoCAD DXF (Drawing eXchange Format) is a CAD data file format developed by Autodesk for enabling data interoperability between AutoCAD and other applications.

DXF was originally introduced in December 1982 as part of AutoCAD 1.0, and was intended to provide an exact representation of the data in the AutoCAD native file format, DWG (Drawing). For many years Autodesk did not publish specifications making correct imports of DXF files difficult. Autodesk now publishes the DXF specifications online.

The more precise cite from the DXF reference itself:

The DXF™ format is a tagged data representation of all the information contained in an AutoCAD® drawing file. Tagged data means that each data element in the file is preceded by an integer number that is called a group code. A group code’s value indicates what type of data element follows. This value also indicates the meaning of a data element for a given object (or record) type. Virtually all user-specified information in a drawing file can be represented in DXF format.

No mention of interoperability between AutoCAD and other applications.

In reality the DXF format was designed to ensure AutoCAD cross-platform compatibility in the early days when different hardware platforms with different binary data formats were used. The name DXF (Drawing eXchange Format) may suggest an universal exchange format, but it is not. It is based on the infrastructure installed by Autodesk products (fonts) and the implementation details of AutoCAD (MTEXT) or on licensed third party technologies (embedded ACIS entities).

For more information about the AutoCAD history see the document: The Autodesk File - Bits of History, Words of Experience by John Walker, founder of Autodesk, Inc. and co-author of AutoCAD.

DXF Reference Quality

The DXF reference is by far no specification nor a standard like the W3C standard for SVG or the ISO standard for PDF.

The reference describes many but not all DXF entities and some basic concepts like the tag structure or the arbitrary axis algorithm. But the existing documentation (reference) is incomplete and partly misleading or wrong. Also missing from the reference are some important parts like the complex relationship between the entities to create higher order structures like block definitions, layouts (model space & paper space) or dynamic blocks to name a few.