Compatibility between Framemaker 12 and latest verstion

Wiebke98A1: … a template in FM12 (unstructured). Is it possible to use this template in FM latest version?
Any later version of FM can open the files of any earlier version of FM. FM2020 (16.0) can open FM12 files.

re: And save it to FM12 (unstructured)?
Yes, but not as .fm or .book. Later FMs can only save-back as binary one generation (for FM2020, that would be to FM2019/15.x).

To hand off to any earlier version, you need to use MIF (Maker Interchange Format),
.AND.
while you work in FM2020, you need to avoid using any features not in FM12, as FM12 will ignore those markups when processing the resulting MIF.

FM12 ⇌ FM16 should be pretty fluid, compared to working with FM7 or earlier (Unicode boundary).