XS is identical to SCH 80 for NPS 1/8 to NPS 8, inclusive, and indicates .500" wall thickness for NPS 8 and larger.
ˌɛksˈɛs ɪz aɪdˈɛntɪkəl tʊ ˌɛssˌiːˈeɪtʃ ˈeɪɾi fɔːɹ ˌɛnpˌiːˈɛs wˈʌn slˈæʃ ˈeɪt tʊ ˌɛnpˌiːˈɛs ˈeɪt, ɪŋklˈuːsɪv, ænd ˈɪndᵻkˌeɪts .fˈaɪvhˈʌndɹɪd" wˈɔːl θˈɪknəs fɔːɹ ˌɛnpˌiːˈɛs ˈeɪt ænd lˈɑːɹdʒɚ.