I found more guides about this over at GameFAQs. The multiple passwords per stage or level (these are synonymous, it's just a bit of internet ambiguity) are used as a rudimentary save-game feature.
There are 6 game states for stage 5, 6, and X. Jennifer is alive, a mindless beast, or dead. David is alive or dead. This gives 18 passwords. Stage 3 and 4 have 3 states for another 6 passwords, while stage 2 has 2 states for another 2.
+----------+--------+--------+----------+--------+--------+--------+
| jennifer | alive | | mindless | | dead | |
| david | alive | dead | alive | dead | alive | dead |
+----------+--------+--------+----------+--------+--------+--------+
| 2 | REISOR | ------ | ------ | ------ | OWDRAT | ------ |
| 3 | ETLBUD | ------ | NAMEPA | ------ | NAIGEP | ------ |
| 4 | TEKROH | ------ | TABRAE | ------ | DABHSN | ------ |
| 5 | RUATNC | ELPOEB | HTRACM | REYALS | NOILCK | TSAEBD |
| 6 | PHENIX | VELGAE | IRTACK | LILITH | DEPITN | URUURU |
| X | HARUON | *** | XASABR | SECOLQ | SARDNA | LEZAJM |
+----------+--------+--------+----------+--------+--------+--------+
***: This is a special case: the guide lists 5 possible passwords:
GOFMTS HCTERG NILVED SORABV ETIRPS
Some testing would be needed as to what the purpose of this is, if there is any. It seems to me these passwords store which stage you came from (this may be any of 1,2,3,4, or 5) so that the game knows where to send you after completing stage X. (So there are really multiple of these bonus levels, one per level but the last.)
That hypothesis would mean stage X should really feature even more passwords: while only HARUON
was listed in the FAQ, there should be at least 5 passwords for that case. At least 4 for XASABR
, at least 5 or SARDNA
, etc.
Perhaps the normal stages also track something about stage X, and that's why there are more possible password states?