Sometimes one plot isn't enough to tell a story and thus the plot arc is born. Plots arcs are discrete plots which, combined together, tell a wider tale, focussing on a particular character or theme.
Some of the plot arcs in X-Project have been:
The Mistra arc by Alicia: Collateral Damage, Trojan Horse, X-Men Mission: Wildchild, Reclaimed, Lost and Found, Thermopylae, X-Men Mission: Loose Ends, X-Men Mission: Phalanx.
The Tomorrow's Children arc by Alicia: X-Men Mission: A Mirror Darkly, Kashmir Waltz, Lost Generations, X-Men Mission: Air Xavier, Skeleton Coast, Trinity, X-Men Mission: Trans-Siberian Hustle, House of Wind.
The Traitor arc by Amy: This Love, This Hate; A Fair To Remember; Traitor; No Way Out.
The The Prince of Thieves arc by Dex: Drop of Nightshade, Wendy, Voodoo Child, New Orleans Is Sinking.
The Lost In The Woods and Hellfire and Damnation arcs by Rossi and Alasdair: Shadows Fall, Little Girl Lost, Buy Now, Pay Later, An Offer You Can Refuse, Just One Drug, High Cost Of Living, Lost In The Woods, Hellfire and Damnation.
The Kingmaker - Phase 1 arc by Dex: Devil In My Mind, X-Men Mission: Alcatraz, Zan Zar Zameen, X-Men Mission: Opera Redux, Prayer on the 26th of July, Gene Nation (plot), Bhagavad Gita.
The Taygetos arc (ongoing) by Alicia: Reunion Monticello, Seven Minutes In Heaven, Krypteia, The Magnificent Seven, Walking Ghosts, Heretics