Contacted tascam with this question,got a reply today it said
"Dear Robin,
Thank you for contacting Tascam U.K
The horizontal line "Level meter guide screen" displayed in <<Meter screen>> of display indicates -12 dBFS.
Please refer to "4. Level meter guide" of owner's manual page 26 "Meter Screen".
Kind regards
Jo Fialho
Tascam U.K Support" They must have a different manual to me , there's nothing about it on page 26 of mine???? Nice to know though.