When an SNMP message is received by this entity,
the community string in the message is compared
with this string second. If it matches, read-write
access is granted to all items in the MIB except
restricted items such as community strings.
(Restricted items appear empty when read and
return a no such name error when an attempt is
made to write them.) If it doesn't match, the
sysCommunityRo string is compared next.
Note that this item is only accessible when using
the community string defined in sysCommunityRwa.