From: Junio C Hamano
Due to the immutable nature of git objects, old objects can be archived into shared, read-only directories. This variable - specifies a ":" separated list of git object directories which - can be used to search for git objects. New objects will not be - written to these directories. + specifies a ":" separated (on Windows ";" separated) list + of git object directories which can be used to search for git + objects. New objects will not be written to these directories.