Ice class is also an endurance question

Are civilian and military ice classifications simply different labels for the same capability?

No. The practical difference includes damage tolerance, endurance in ice, and how long the vessel can operate before maintenance.

The classification depends on tolerated damage, operating duration, and maintenance burden. Two ratings may permit work in broadly similar ice while assuming different exposure times. A civilian vessel may need only a limited seasonal capability before returning for upkeep. Military planning allows for longer use and for conditions to become more demanding or less predictable.