When "Available for use across all partitions (Global Access)" is selected, any partition can use this account and the password is given to other administrators. For example, this account can be used as a dependent account or a service account for other assets. Potentially, you may have assets that are running services as the account, and you can update those assets when the service account changes.
If not selected, partition owners and other partitions will not know the account exists. Although archive servers are not bound by partitions, this option must be selected for the directory account for the archive server to be configured with the directory account.
Based on architectural changes made from 2.x to 6.x, this flag was added to allow one service account to manage assets across multiple partitions. The consequence is that the account is visible to users with the Asset Management view. However, only general information is available about the account, and no check/change/set operations are permitted.