afconvert: Difference between AAC formats: 'aac ' 'aace' 'aacf' 'aach' 'aacl' 'aacp' 'alac' 'paac'
The "names" stem from the four char values defined in the Software Development Kit for CoreAudio. Specifically, they are from the header CoreAudioTypes.h. You can find the documentation here.

enum {
kAudioFormatLinearPCM               = 'lpcm',
kAudioFormatAC3                     = 'ac-3',
kAudioFormat60958AC3                = 'cac3',
kAudioFormatAppleIMA4               = 'ima4',
kAudioFormatMPEG4AAC                = 'aac ',
kAudioFormatMPEG4CELP               = 'celp',
kAudioFormatMPEG4HVXC               = 'hvxc',
kAudioFormatMPEG4TwinVQ             = 'twvq',
kAudioFormatMACE3                   = 'MAC3',
kAudioFormatMACE6                   = 'MAC6',
kAudioFormatULaw                    = 'ulaw',
kAudioFormatALaw                    = 'alaw',
kAudioFormatQDesign                 = 'QDMC',
kAudioFormatQDesign2                = 'QDM2',
kAudioFormatQUALCOMM                = 'Qclp',
kAudioFormatMPEGLayer1              = '.mp1',
kAudioFormatMPEGLayer2              = '.mp2',
kAudioFormatMPEGLayer3              = '.mp3',
kAudioFormatTimeCode                = 'time',
kAudioFormatMIDIStream              = 'midi',
kAudioFormatParameterValueStream    = 'apvs',
kAudioFormatAppleLossless           = 'alac'
kAudioFormatMPEG4AAC_HE             = 'aach',
kAudioFormatMPEG4AAC_LD             = 'aacl',
kAudioFormatMPEG4AAC_ELD            = 'aace',
kAudioFormatMPEG4AAC_ELD_SBR        = 'aacf',
kAudioFormatMPEG4AAC_HE_V2          = 'aacp',
kAudioFormatMPEG4AAC_Spatial        = 'aacs',
kAudioFormatAMR                     = 'samr',
kAudioFormatAudible                 = 'AUDB',
kAudioFormatiLBC                    = 'ilbc',
kAudioFormatDVIIntelIMA             = 0x6D730011,
kAudioFormatMicrosoftGSM            = 0x6D730031,
kAudioFormatAES3                    = 'aes3'
};