Here is a list of all files with brief descriptions:

[detail level 123]

docs
configuration
modules
include
atmega328p
ADC.h	ADC channels, references, timing, and electrical characteristics
AnalogComparator.h	Analog comparator inputs and operating modes
Architecture.h	AVR8 architecture characteristics
ATmega328P.h	Aggregate include for all ATmega328P definitions
Bootloader.h	Boot section and Flash region definitions
Clock.h	Clock sources, startup settings, and prescalers
Config.h	Selects the ATmega328P package and operating grade
CPU.h	CPU registers, pointer pairs, and status flags
DebugWire.h	DebugWIRE pin and fuse characteristics
Device.h	Device identity and peripheral counts
EEPROM.h	EEPROM geometry and programming modes
Electrical.h	Electrical limits and operating-grade profiles
Errata.h	Silicon revision and errata references
ExternalInterrupts.h	External interrupt lines and pin-change groups
Fuse.h	Fuse byte fields and factory reference values
GPIO.h	GPIO port characteristics and hardware limitations
Interrupts.h	Interrupt sources, vector numbers, and program addresses
LockBits.h	Flash and boot lock-bit definitions
Memory.h	Flash, SRAM, EEPROM, and data-space geometry
Package.h	Package characteristics and selected physical package
PackagePins.h	Physical terminal numbers for each ATmega328P package
Pin.h	Logical GPIO pin and port identifiers
PinFunctions.h	Pin capabilities and alternate peripheral functions
Power.h	Sleep modes and peripheral power reduction
Programming.h	Programming interfaces and device programming metadata
PWM.h	PWM channels, timers, and resolutions
QTouch.h	QTouch software-support characteristics
RegisterBits.h	Register bit positions and field masks
RegisterMap.h	Data-space addresses of the ATmega328P registers
Reset.h	Reset sources and brown-out characteristics
RTC.h	Timer2 asynchronous clock characteristics for RTC use
SPI.h	SPI modes, roles, and clock settings
Timer.h	Timer/counter modes, widths, and clock selections
TWI.h	Two-wire interface status codes and bus limits
USART.h	USART modes and frame-format definitions
Watchdog.h	Watchdog oscillator and timeout characteristics