@werner:

See this example where I change the trust value to "2" as the program currently
enables (with one field):

$ echo "ABCDABCD....ABCD:3:" | gpg --import-ownertrust
gpg: changing ownertrust from 6 to 3
$ gpg --edit-key "Dor"
gpg (GnuPG) 1.4.10; Copyright (C) 2008 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

gpg: checking the trustdb
gpg: no ultimately trusted keys found
pub <key_size>R/<key_id> created: 2013-09-25 expires: never usage: SC

trust: never         validity: unknown

sub <key_size>R/<key_id> created: 2013-09-25 expires: never usage: E
[ unknown] (1). <Full name> (<nickname>) <email@addr.com>

---------------

There's a string written "unknown" - why is that? Maybe something's missing?

See another example when I set the trust value to Ultimate with two fields:

$ echo "ABCDABCD....ABCD:6:6" | gpg --import-ownertrust
gpg: changing ownertrust from 3 to 6
$ gpg --edit-key "Dor"
gpg (GnuPG) 1.4.10; Copyright (C) 2008 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Secret key is available.

gpg: checking the trustdb
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u
pub <key_size>R/<key_id> created: 2013-09-25 expires: never usage: SC

trust: ultimate      validity: ultimate

sub <key_size>R/<key_id> created: 2013-09-25 expires: never usage: E
[ultimate] (1). <Full name> (<nickname>) <email@addr.com>

---------------

Now we can see the string "ultimate" instead of "unknown".

This is why I think there are 2 fields in the --export-ownertrust command.