Bipolar electrolytic capacitors have 2 types of construction possible: "rough" or "smooth".
The difference is the method of manufacturing the aluminum foil used to wind the capacitor.
It is possible to make the sheet rough, so the equivalent surface area is increased.
The capacitance of the capacitor is proportional to the surface area of the aluminum foil, and by this process larger capacities can be obtained with the same amount of material.
Thes bipolar capacitors are therefore recommended in applications requiring large capacities and / or for a question of costs, the capacitors with sheet (Type MCap®) are forbidden.