"I have a question regarding ..."
In English, to say you have a doubt implies that you do not believe
something. In this case, you have a question, not a doubt. In answer
to your question, the file format for isosurfaces is unique to
Smokeview. The format of this file is given in the Appendix of the FDS
User's Guide, because it is FDS, not Smokeview, that creates the file.