Being pilot in command (PIC) comes with awesome power (except in my Aeronca Champ) and with that a certain amount of discretion. When ATC says, "Cirrus 345, descend at pilot's discretion, maintain 5000," the discreet pilot knows that ATC has authorized a decent at will, at any rate, plus the pilot may temporarily level off at any point -- perhaps to admire the sunset -- and even return to the original altitude.
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