This object reports the actual line number where this ancillary service was found in the bottom field of an interlaced SDI signal, as opposed to F1's top-field reading. For interlaced video, ancillary data can appear on different physical lines in each field, so an admin verifying full-signal integrity checks both F1 and F2 to confirm the service is being correctly carried in both fields, not just one. A source with a subtle field-timing issue might show the service present with a normal line number on F1 but a wildly different or missing value on F2, tipping off an admin to a bottom-field-specific signal problem.