Here is a list of all documented namespace variables with links to the namespaces they belong to:

ICONV_E2BIG : SDL
ICONV_EILSEQ : SDL
ICONV_EINVAL : SDL
ICONV_ERROR : SDL
IDENTIFIER_STRING : SDL::prop::appMetaData
IGNORE_PROPS_BOOLEAN : SDL::prop::AnimationDecoder::Metadata
IMAGE_ALPHA : SDL
IMAGE_COLOR : SDL
IMAGE_INVALID : SDL
IMAGE_SDF : SDL
INIT_AUDIO : SDL
INIT_CAMERA : SDL
INIT_EVENTS : SDL
INIT_GAMEPAD : SDL
INIT_HAPTIC : SDL
INIT_JOYSTICK : SDL
INIT_SENSOR : SDL
INIT_STATUS_INITIALIZED : SDL
INIT_STATUS_INITIALIZING : SDL
INIT_STATUS_UNINITIALIZED : SDL
INIT_STATUS_UNINITIALIZING : SDL
INIT_VIDEO : SDL
INVALID_UNICODE_CODEPOINT : SDL
IO_SEEK_CUR : SDL
IO_SEEK_END : SDL
IO_SEEK_SET : SDL
IO_STATUS_EOF : SDL
IO_STATUS_ERROR : SDL
IO_STATUS_NOT_READY : SDL
IO_STATUS_READONLY : SDL
IO_STATUS_READY : SDL
IO_STATUS_WRITEONLY : SDL
IOSTREAM_AUTOCLOSE_BOOLEAN : SDL::prop::AnimationDecoder::Create, SDL::prop::AnimationEncoder::Create, SDL::prop::Font::Create
IOSTREAM_OFFSET_NUMBER : SDL::prop::Font::Create
IOSTREAM_POINTER : SDL::prop::AnimationDecoder::Create, SDL::prop::AnimationEncoder::Create, SDL::prop::Font::Create