Holds Selection Depth is the field that controls which range of libraries Evergreen will target a copy for a hold, which is useful information for troubleshooting a hold. The field is filled by Evergreen when the hold is placed.
Selection depth 2 means within your regional federation or BC ILC or Spruce, 3 means within your library or its branches only. For a hold with selection depth of 3, Evergreen will not target other regional libraries' copies. This is how Evergreen makes it possible not to target other libraries' copy when your library has a copy.
The hold targeter will not target subsequent new copies so staff may need to cancel and re-target the hold on a particular copy to update the selection depth in situations where hold will not be filled otherwise.