Newbie question about DFX vs. STL

I'm just learning about this stuff and I understand that DFX files
are used mainly for 2D drawings and STL for 3D. What exactly is the
difference? I'm trying to read between the lines (metaphorically)and
am beginning to believe that DFX is used in traditional programs like
Autocad and the like. I'm also learning Autocad right now and am I
correct in saying that DFX is used more for dimensionally derived
drawings where a part must meet certain preconcieved dimensions? You
know, parts as part of an assembly or that uses standardized threaded
parts etc. I haven't actually read this info anywhere but have just
gathered this over time? Am I right?

Conversely is STL used mainly for prototyping of shapes and models
that are complete unto themselves? That is don't have to meet rigid
dimensions but rather must have all parts scaled to one another based
on an arbitrary scale?

I'd sure like to know if what I've just said is right or wrong? It
would definitely help me in knowing which direction I should be
learning. Most of what I do will fit in the category of parts that
need to fit with other parts that are already dimensionally specified.
Thanks for any info this group can give.

Eric