Variable font naming

Today I made my first variable font. And it works in Adobe Illustrator! Great to see it work.
But I have a question about the naming of the font. The font has the name that I chose as Family Name, but the letters GX are added. In Illustrator these added letters don’t show up in the font menu. Why are these letters added in the font file name?
Another question: in the export window is written that an OpenType Variable font will be exported. But the file has the extension .ttf. Is it an OpenType font or a True Type font?

GX: This is only the file name.
TrueType is one kind of OpenType.