Monster Virtualization is the process of Monsters spawning. There are variety of ways that monsters spawn.
Standard Monster Virtualization is by far the most common form of monster virtualization shown. This is usually run by X.A.N.A.. The monster's vector lines appear, and then the area between the vector lines is filled, and the monster either falls to the ground or flies.
Spawning From The Ground
Manta Mine Laying
On two occasions, X.A.N.A. materialized monsters via the Scanners. This first happened in "False Start", where X.A.N.A. materialized a small army of Kankrelats. Later, in "A Bad Turn", three Krabs were materialized, though doing so caused the scanners to be destroyed.
The Kolossus has been virtualized thrice and they had the same, albeit unique, procedure. In this variation, hundreds of Replikas start allocating their respective resources to the creation of the Kolossus. It then emerges from that Sector's Digital Sea. The first usage of this revealed the sheer number of replikas to the Lyoko Warriors, which led Jeremie to develop the Anti-X.A.N.A. Program.