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

John Theune writes:

Are you flying IFR?

Yes. I've filed a SID, a route, and a STAR, and programmed this into
the FMC as well. The FMC apparently decides when to start the descent
from cruise (if you don't override it), presumably based on the
altitude restrictions it has to respect for the arrival procedure.

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 (?).

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