A ship advertised for polar travel carries an ice class, and that rating is a structural certification rather than a marketing term. It sets the limits of where and when the vessel may operate.
What the rating actually describes
Ice class covers the thickness of hull plating in the ice belt around the waterline, the spacing and strength of internal framing, and the protection of rudders and propellers.
It also covers machinery. A vessel operating in ice needs the power to keep way on against resistance and cooling water intakes that will not block with slush.
Classification societies assign the ratings, and they are graded steps rather than a single threshold, ranging from vessels that may meet occasional loose ice to true icebreakers.
Higher class does not mean icebreaker
Most polar passenger ships are strengthened to transit ice, not to break a channel through it. They are designed to work in leads and open water between floes.
An icebreaker has a hull form that rides up onto ice and uses weight to fracture it, along with the power and reinforcement to absorb repeated impact. Very few passenger vessels are built that way.
The distinction matters for itineraries. A strengthened ship needs a navigable route to exist; an icebreaker can, within limits, make one.
The polar code adds operational limits
Beyond structure, international rules for polar operation require assessment of a ship's capability against expected ice conditions, and require a certificate reflecting that assessment.
The same framework covers survival equipment, crew training and the ability to sustain everyone aboard for an extended period if assistance is far away.
Because rescue resources in polar regions are thin, the rules push operators toward conservative planning, which is one reason seasons are short and routes are limited.
Ice is seasonal, not fixed
Sailing seasons in both polar regions are set by when the ice retreats enough for a given class of ship, and they close when it returns. That window governs when itineraries are offered.
Conditions vary year to year, so an operator plans a route knowing that a particular strait may or may not be passable when the ship arrives.
Ice charts are consulted daily during a voyage, and a route that was viable at departure can close behind the ship, which is why alternates are always carried.
What a traveler should read into it
A stated ice class tells you what kind of water the ship is certified for, which is more informative than any description of the itinerary as remote or adventurous.
It also predicts the ride. A strengthened hull with the form needed for ice tends to roll more in open water, which matters on crossings to reach the ice.
Finally it explains cost. Ice strengthening, redundant machinery, higher crew ratios and short seasons all load onto a small passenger count, which is why these voyages price the way they do.