https://en.wikipedia.org/wiki/Global_Release_Identifier
https://www.ifpi.org/resource/grid/
https://www.ifpi.org/wp-content/uploads/2020/08/GRid_Standard_v2_1.pdf

In short, GRid are standardized identifiers for digital media that was originally developed by authoritative organizations within the music business such as IFPI and RIAA. They are supposed to play key role in identifying and differentiating digital releases distributed for streaming and paid download across different platforms (Spotify, Apple Music, etc.).

At the time of me posting this ticket, one digital vendor that features GRid most prominently is the Japanese platform MORA which has them embedded in the URLs for their release pages. This practice however isn't uniform because often the role of unique identifiers of MORA releases can be fulfilled by the UPC (barcode) or labels' own internal catalog IDs instead.

A few other vendors might either report them through their API (e.g. HDTracks) or have this identifier hidden in the HTML source of the release pages (usually inside a block of JSON metadata for the release in question as its the case with Beatport and Beatsource).

Without a dedicated GRid field for releases, the practice of including this info into MB entries has so far been a mess. Some editors have entered them inside the ''catalog-number'' field, whereas others have opted to only mention them within the annotation instead.