BGP communitiy can be used to as tag of a set of routes. Those tags can be used for applying specific routing policy. Standard community does not affect best path selection of the local router.
The extended community ， which is called cost community，influences the BGP best path selection process at the point of insertion（POI）.By default，the POI follows the IGP metric comparison.Cost community is configured with a cost community ID and cost number.Path with the lowest cost community number will be prefered.If the cost community number is the same，path with the lowest cost community ID is prefered.