Traced references are tracked by whatever form the language offers automatic memory management.
Untraced references are a kind of safe pointers, basically the memory they point to isn't tracked by the GC/RC infrastructure and they are managed in some form of manual memory management, and can be used as part of unsafe code for pointer arithmetic.
The languages then provide means of converting between both worlds, naturally it always requires making use of some form of unsafe code block.
I thought it might be an Oberon specific concept. I'd like to see something like ref counting, but an actual ref list where there is a global list of references to an object.
What it would be missing, and was fixed in Active Oberon, is explicit support for untraced references.