Confusion about when it's my navigation, and when it's ATC

In a previous article, Mxsmanic said:
if so then ATC will tell you what altitude to be at.
Resume own navigation refers to routing not altitude.

Ah ... is there another phrase that also means altitude is at my
discretion, or is altitude always under the control of ATC? The fact
that the FMC provides for its own descent schedule implies that there
must be situations in IFR where I'm allowed to climb or descend at my
discretion (?).

If your FMC wants to descend, ask ATC for a descent. If you're going to
be descending with multiple segments of different descent rates, ask ATC
for "descent at pilots discretion".

Real world IFR flying is about coordinating your actions with those of all
the other planes out there, not flying along fat, dumb and happy doing
whatever you feel like. That's the whole point of ATC - to make sure you
don't hit what you can't see. ATC's second priority is to make sure the
system runs smoothly, so your actions don't interfere with somebody else
and somebody else's actions don't interfere with you.

--
Paul Tomblin http://blog.xcski.com/
"I love the smell of burning components in the morning.
Smells like victory." (The ******* Operator From Hell)