From The PSPedia2
Urgent missions are special missions that are not always available to the player, and the equivalent of PSU's rare missions. The right to play an urgent mission is earned by completing free missions in multi mode or internet multi mode only (they are not available in story mode). Urgent missions do not appear after completing story or extra missions.
The chance of an urgent mission appearing is based on the number of kills accrued in the course of playing the free mission. More kills results in a higher chance to get an urgent mission. Furthermore, the difficulty of the mission is the same as that of the free mission. As a result, it is not a good idea to race through C-rank free missions to try and earn urgent missions.
Note that the chance of getting an urgent mission is not directly affected by the number of players in the party, and it is possible to get urgent missions while playing solo in multiplayer modes.
Rare drop rates are tripled in urgent missions, and rare enemies appear frequently. It is possible to get two urgent missions in a row. (example: Flower of Flames appeared after clearing Phantom Ruins)
PSO urgent missions
These missions take place in the old PSO areas, and are similar to the rare missions found in PSU. Each mission is available only by playing free missions on a particular planet.
- Phantom Ruins - 幻影の聖跡 (Clad 6 only)
- Illusionary Shaft - 幻影の横坑 (Parum only)
- Phantom Fissure - 幻影の火孔 (Neudaiz only)
- Forest of Illusion - 幻影の樹林 (Moatoob only)
SEED purification missions
These missions originally appeared as event missions in PSU's Operation: Firebreak and Absolute Zero events. They can appear on any planet.