This modifier is documented in Lesk's tbl paper in the Seventh Edition Unix manual (1979) and is supported by DWB 3.3 troff, upon which Heirloom Doctools is based.
$ cd ~/dwb
$ cat table-with-z-column-modifier.roff
.TS
tab(@);
L Lz L.
one@/@two
.TE
$ DWBHOME=. ./bin/tbl table-with-z-column-modifier.roff | DWBHOME=. ./bin/nroff | cat -s
one / two
$ cd ~/heirloom
$ ./bin/tbl table-with-z-column-modifier.roff | ./bin/nroff | cat -s
table-with-z-column-modifier.roff: line 3: bad table specification character
tbl quits
L.
one@/@two
This modifier is documented in Lesk's tbl paper in the Seventh Edition Unix manual (1979) and is supported by DWB 3.3 troff, upon which Heirloom Doctools is based.