HIV transmissibility relates to whether someone has been on HAART therapy and in compliance with that therapy long enough so that their measured viral load as been assessed and has dropped below a particular threshold.
Transmissibility also relates to transmission vectors: eg vaginal or anal intercourse, sharing needles with intravenous drugs, and infected-mother-to-child routes (eg placental and/or breastfeeding).
In particular the "Undetectable = Untransmissible" claim is (as I understand it) a claim specifically about transmission through vaginal and anal intercourse.
Transmissibility also relates to transmission vectors: eg vaginal or anal intercourse, sharing needles with intravenous drugs, and infected-mother-to-child routes (eg placental and/or breastfeeding).
In particular the "Undetectable = Untransmissible" claim is (as I understand it) a claim specifically about transmission through vaginal and anal intercourse.