The reason for protected poles is to provide the pole with an overcurrent release. An unprotected pole is a pole that does not have overcurrent release

The first number corresponds to the number of poles.
The second number corresponds to the number of protected poles.
The first letter stands for Pole.
The second letter stands for trip unit ("déclencheur" in French).

Examples:
4P3d is a 4-pole trip unit with 3 protected poles (not the neutral).
4P4d is a 4-pole trip unit with 4 protected poles (including the neutral).
4P3d+N/2 is a 4-pole trip unit with 3 protected poles and the neutral at half rating due to the neutral cable being half the phase cable cross sectional area.
4P3d+OSN is a 4-pole trip unit with 3 protected poles and with oversized protection of the neutral (Over Size Neutral).

Released for: Schneider Electric India