Originally communities were Cisco proprietary treating the 32 bit COMMUNITY as a decimal value. When it was added as a PA, and the standard RFC it was formatted as AA:NN where AA is the 16 bit number to represent an ASN and NN is a value as set by that ASN, however still a 32 bit number
By default routers will display the community in its decimal format, yo uhave to globally enable the new format for it to display instead. Use the ip bgp-community new-format command for this.
You can set multiple communities on NLRIs where the policy applies and you can add to existing communities using the additive option command.
Extended communities (100-199) allow for matching via regex.
Community lists can be edited and certain values removed with the use of route maps as well.