This is addressed in the film's official novelisation. The orders are that the mission objective (stopping Shinzon) is their primary concern and that their own lives and the wellbeing of the ship and crew are secondary to that objective.
"We can only hope so." Picard paused to somberly study each of his officers in turn. "He can't be allowed to use that weapon. All other concerns are secondary. Do you understand me?"
From the grim expressions around the table, it was clear each person did. The Enterprise - and all aboard her - were expendable.
Riker spoke for all of them. "Yes, sir."
You may wish to note that this is far from the first time that a crewman (or even an entire crew) has been told that their lives are expendable. Troi orders (holo)Geordi to his (holo)death in TNG: Thine Own Self, the entire Voyager, and indeed the Prime Directive itself, is put on notice in VOY: The Omega Directive, and Kirk makes it clear that he's been told to avoid war at all costs in TOS: Balance of Terror:
KIRK: But if necessary to avoid interspace war, both these outposts and this vessel will be considered expendable. Captain out.