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 full enhanced and Aptitude based rules, which is an Advanced concept.
The “essential” materials reflect the current set of i20™ Branch Essentials for D20 System of © DnD 5th, 5.5, and OneDnD Editions ™. All mechanic changes reflect the intended “gritty realism” of Incarna (initially depicted in the 5E DMG p.267). This is represented as an increased level of lethality and danger inherent in combat and confrontation, and difficulty in acquiring and using pertinent knowledge and capability. It also uses the Incarna scaled result based on the Rule of Averages (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 from the difficulty (DC) 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).