Maximus HD is a custom ROM and built on top of AOSP code by LlabTooFeR with some added features on top of AOSP. This is the Maximus HD ROM features page that notes of features added in Maximus HD ROMs.
Maximus HD ROM features for HTC One M9:
Based on HTC One (M9) 2.7.401.1
Android 5.1 (LMY47O)
Sense 7.0
Amazing battery life
Fast and Stable
Tweaked for Best Performance
OTA Updates
Auto-Kernel Flash
Advanced Power Menu
Multi User support
Display Color Calibration
OpenGL ES 3.0
Deodexed
Zippaligned
Rooted
Aroma Multi-Language Setup (Supports 26 languages now)
Option to remove unneeded apps
Option to install additional mods
Support of init.d
Always newest ROM bases