A reference is a copy of an asset’s video or audio content that YouTube uses for Content ID matching. It is one of three representations of your intellectual property in the YouTube system. It is not a publicly viewable YouTube video, but rather a representation of an asset in YouTube’s Content ID system.
From a reference, YouTube creates a digital fingerprint that Content ID compares with videos uploaded by other YouTube users. When Content ID detects a match, it automatically claims the video on your behalf and applies your designated match policy to it.
An asset can have more than one reference. For example, a movie asset could have separate references with 16:9 and 4:3 aspect ratios. The reference need not be the same as a viewable YouTube video; see Choose effective references for more information.