This setting on the Server Configuration document controls how the signature is selected.
By default (when this setting is not checked), when determining which Signature Configuration document has been assigned to a user, the application checks each signature configuration document in turn and checks the members of the group.
If the group contains additional groups, then these are checked.
This allows all groups to be used in signature selection but if there are many signature configuration documents and large amounts of nested groups, then this can result in poor performance.
New User Lookup Setting
This new setting provides an alternative method of Signature Configuration selection. Instead of checking each group, it uses the ($ServerAccess) view. This returns the groups the user is a member of.
This method performs a lot better than the default BUT the $ServerAccess view does not list all groups. Mail Only groups are NOT included in the $ServerAccess view so cannot be used for Signature Configuration Selection