This approach to using the d20 variant for Incarna uses a smaller set of essential rules on top of the d20 [SRD] rules to get the Incarna flavor with them. This leaves out the larger set of Aptitude based rules, which is an Advanced concept.
The essential Incarna D20 (D20 System 5th Edition ©DnD™) modifications for Incarna. All mechanic changes reflect the intended “gritty realism” of Incarna (see 5E DMG p.267). This is represented as an increased level of lethality and danger inherent in combat, damage and confrontation, and difficulty in acquiring and using pertinent knowledge and capability. It also uses a scaled result (a single check denoting Simple/Average, Major/Full, and Critical successes); This speeds up resolution, reduces dice dependency, and puts the emphasis on quality of success (comparing how far above the DC [difficulty] required for the action to the result of a check) as opposed to random dice rolls. Items and powers generally have an impact listed for these (ex: damage is 4/8/16 for average/full (Major result)/double full (Critical result).