BGP Extended Community
An Extended BGP community is simply a redefining of the standard BGP community field in the BGP header that allows a 64-bit number to represent the community rather than a 32-bit number. This simply gives more granularity to the configuration of various BGP attributes.
In particular, extended BGP communities provide structure for various classes of information and are very often used for VPN services.
Links:
https://forum.networklessons.com/t/bgp-communities-explained/1239/86?u=lagapides
https://www.ciscopress.com/articles/article.asp?p=2756480&seqNum=12