The update on May 22nd, 2008 introduced PvP versions for some skills. The game replaces the PvE version with the PvP version on the player and his/her heroes whenever the player travels from a PvE area to a PvP area (such as the Guild Hall) and vice versa. A note of this is written to the chat log whenever it occurs, listing the skills and heroes concerned. In addition, the PvP effect is shown in the player's effect monitor when they are in a PvP outpost or in their Guild Hall.
This feature allows PvP- or PvE-specific balance changes to be implemented on the relevant version of the skill without affecting the other version.
On GuildWiki, the base article for each skill (e.g. Shadow Form) currently details the PvE version of the skill, while the PvP version is on a separate page (Shadow Form (PvP)). A disambiguation message at the top of each page links to the other version.
For a list of the PvP versions of split skills, see Category:PvP skills.