"T1 RM = "T1 Realistic Melee": This is human against human with real-life limitations. {e.g. human capabilities for running, jumping, reaction time, body integrity, vision, etc.} Ex: Street Fighting.
T1 UM = "T1 Unrealistic Melee": This is a style that is above-average human against human that are capable of using things like magic, sorcery, vampirism, werewolfism. Ex: Underworld.
*T1 MP = "T1 Moderate Powers": This is the style that is commonly used now as a more realistic look towards T1 PC. Characters that have special powers, just not ones that have a power that is capable of destroying an entire planet. Powers such as elemental abilities are acceptable, but if there is a power that is capable of using all elements you must keep that power in a moderate use. Ex: Bleach, and Naruto.
T1 PC = "T1 Powered Characters": This is the style that is most difficult to learn. The complexity of the character's abilities and powers are what get them called "God-Modders". The depth put into one character, with the powers, abilities, explanations of terrain, dimension, sub-abilities, character background, and traits will lead an inexperienced opponent to believe there is an invincibility about the character."
*T1 MP is the most commonly used style of T1 roleplaying. Very seldom will you find a T1 PC roleplayer that is doing it correctly, and even then, most prefer T1 MP roleplaying.
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T1 Full Rule Set
RandomEverything you need to know about T1 Roleplaying. (Credit goes to Rj Gentry's "T1 Rules and Guidelines for the Modern Day Role Player" on Scribd), parts are taken from "Role Play 101" as well.