CUE is a text file that describes how tracks are arranged in a super-audio CD and DVDs. Typically, CUE sheets come in plain text and have a filename extension of .cue. Today, this format is prevalent in most media players and optical disc authoring apps.
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M4R is a filename extension for AAC format, which is developed by Apple Inc. It is typically used to save custom iPhone ringtones from Apple iTunes. M4R can only be created from iTunes purchased songs and is usually DRM protected. It can open on iTunes, VLC Media Player, Apple QuickTime Player, and more.