a <- lapply(
  X = split(dm[1:3], f = seq_len(nrow(dm))),
  FUN = Domain
)

do.call(rbind, lapply(a, slot, "df"))
#>     STUDYID DOMAIN        USUBJID
#> 1 S-CDSK-01     DM CDISC01.100008
#> 2 S-CDSK-01     DM CDISC01.100014
#> 3 S-CDSK-01     DM CDISC01.200001
#> 4 S-CDSK-01     DM CDISC01.200002
#> 5 S-CDSK-01     DM CDISC01.200005
as.data.frame(t(unique(t(do.call(cbind, lapply(a, slot, "columns"))))))
#>           names
#> STUDYID STUDYID
#> DOMAIN   DOMAIN
#> USUBJID USUBJID