Does somebody know, what .igs means?
I shall create a file for Cathia, what understands .igs-files, but I don't know, if it's the same.

I can assure you that Catia will not "understand" Indigo files!
If I remember correctly .igs stands for "indigo scene"

.IGS stands for Indigo Scene Format.
.IGS is also a very popular NURBS format used to go between NURBS and solids-based programs.

Also noted IGES sometimes if I'm correct.

The IGS files issued from the exporter you are using are to be opened by Indigo as Indigo scenes. IG(E)S files as described by StompinTom will not be recognized by Indigo.

Thanks to all for the answers.