The TYPE tag is a 32-bit unsigned integer (uint32_t) that, when viewed in little-endian format, states the type of firmware image the IMG2/IMG3/IMG4/IM4P file is. This value  is passed to load_selected_image and then image_load early in the SecureROM boot process, where a non-NULL pointer check is performed.
Types
IMG2
IMG3+
| Name | Type | Notes | 
| Any | 0x00 | Represents any image type | 
| ACIBT | acib | ACI Bluetooth | 
| ACIWIFI | aciw | ACI WiFi | 
| Alamo | almo | 
| ANE Firmware | anef | 
| AOP | aopf | 
| AppleLogo | logo | 
| Audio Codec Firmware | acfw | 
| AVE Firmware | avef | 
| BatteryCharging0 | chg0 | Charging indicator (bright) | 
| BatteryCharging1 | chg1 | Charging Indicator (dim) | 
| BatteryFull | batF | 
| BatteryLow0 | bat0 | Empty battery | 
| BatteryLow1 | bat1 | Red composed onto empty battery | 
| CFELoader | cfel | Silicon Validation [1] CFE loader | 
| Dali | dali | 
| DeviceTree | dtre | 
| Diags | diag | 
| EngineeringTrustCache | dtrs | 
| Environment Variables | ienv | 
| FDR Trust Object for AP | fdrt | 
| FDR Trust Object for SEP | fdrs | 
| GlyphCharging | glyC | 
| GlyphPlugin | glyP | 
| GPU Firmware | gfxf | 
| Homer | homr | 
| Hypervisor | hypr | 
| iBEC | ibec | 
| iBoot | ibot | 
| iBootTest | itst | 
| iBSS | ibss | 
| Input Device | ipdf | 
| iSpoof(?) | ispf | 
| KernelCache | krnl | 
| LeapHaptics | lphp | 
| LLB | illb | 
| LoadableTrustCache | ltrs | 
| LowPowerWallet0 | lpw0 | 
| LowPowerWallet1 | lpw1 | 
| LowPowerWallet2 | lpw2 | 
| Mac EFI | mefi | 
| Maggie Firmware | magg | 
| Multitouch Firmware | mtfw | 
| NeedService | nsrv | 
| OSRamdisk | osrd | 
| PEHammer | hmmr | PE Hammer test | 
| PersonalizedDMG | pdmg | 
| PERTOS | pert | PE RTOS environment | 
| PHLEET | phlt | Silicon Validation PHLEET test | 
| PMP | pmpf | 
| RestoreRamDisk | rdsk | 
| RBM | rbmt | Silicon Validation RBM test | 
| RecoveryMode | recm | 
| RestoreDeviceTree | rdtr | 
| RestoreKernelCache | rkrn | 
| RestoreLogo | rlgo | 
| RestoreOS | rosi | 
| RestoreSEP | rsep | rsep is also present in sepboot | 
| RestoreTrustCache | rtsc | 
| RTP | rtpf | 
| SCE | scef | 
| SEP | sepi | 
| SmartIOFirmware | siof | 
| StaticTrustCache | trst | 
| System Locker | lckr | 
| TSYS Tester | tsys | 
| WCHFirmwareUpdater | wchf | 
| Wireless Charging | wchf | 
References
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