Looking for info on debconf-set-selections/pre-configuration/pre-seeding specs format

Question

The output of debconf-get-selection, once the comment and blank lines are filtered out, consists of records (i.e. rows) of 4 (or sometimes 3) whitespace-separated fields. For example¹:

d-i                 apt-setup/services-select          multiselect  security, updates
d-i                 apt-setup/security_host            string       security.debian.org
tasksel             tasksel/first                      multiselect  standard, web-server
tasksel             tasksel/desktop                    multiselect  kde, xfce
popularity-contest  popularity-contest/participate     boolean      false
d-i                 finish-install/reboot_in_progress  note

This 4-field format is also the one prescribed a for "pre-configuration" (aka "preseed") file. (Such a file may be passed to the Debian installer to automate some or all of a Debian installation.)

Hence one could say that a file in this 4-field format has two slightly different possible interpretations: it can either be a report of an existing Debian-based system, or it can be a set of directives for the Debian installer.

AFAICT, values for the first three fields in this format are "controlled" (i.e. they come from some prescribed, controlled vocabulary), whereas values for the last field are, in general, "uncontrolled" (aka "free text").

For example, these are all the field-3 entries I've come across so far:

boolean select string error note multiselect password terminal entropy

In contrast, there's a lot more variety and heterogeneity among the field-4 entries; they alone can include whitespace, or be omitted entirely (in which case the preceding delimiter may also be omitted, AFAICT; BTW, the last record in the excerpt above illustrates the case where the fourth field is missing). Field-4 entries I've seen include

some number
Configure network manually
r00tme
wlan: 0 Atheros Communications Inc. AR9462 Wireless Network Adapter (wireless)
most
None
50some_device__________regular
local delivery only; not on a network
Zurich
Finish the installation

...etc.

I'm looking for more details than I've been able to get from the man pages for debconf-set-selections and debconf-get-selections on the format/schema above. In particular, I'm looking for answers to the following questions:

1. what are the possible values for each of the controlled fields (fields 1, 2, and 3), and what do these values mean, precisely?
2. what does it mean when the fourth field is empty? and more specifically,
   1. what is debconf-get-selections reporting about an installed system with a record whose fourth field is empty?
   2. how does the Debian installer interpret a record whose fourth field is empty?

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_{¹Note that, for the sake of readability, I've used whitespace in this excerpt in a way that does not adhere to the specifications for the debconf-get-selections/debconf-set-selections format; the latter state that a single space or tab should be used to separate the fields.}