Keys in ini files
(Under development.)
Many keys are related to the CLDR (Common Language Data Repository). Others are just the TeX primitives with the same name.
identification
Most of them are self explanatory.
charset The charset in the ini file (currently must be utf8).
tag.bcp47 May includes if appropriate language, script and region. Usually only the language.
language.tag.bcp47 Th language part.
tag.bcp47.likely The likely full tag. See Likely Subtags (CLDR)
tag.opentype
script.name
script.tag.bcp47
script.tag.opentype
level inifiles are based on a set of keys. The level is much a
‘version’ of the list of available keys. Currently is 1, and it will
stay so until there is some significant change.
derivate Not yet used, but its purpose is to identify if the files
is the original one distributed with babel or a derivate (for
example, a publishing house may want to define its own files).
encodings A mostly informative field for 8-bit engines requiring
font encodings (T1, LGR, etc.)
captions
The .licr subsections are used in 8-bit engines. The final name is
added by babel.
date
Here are some explanations for dates:
http://cldr.unicode.org/translation/date-time-names
For example, about narrow:
The narrow date fields are the shortest possible names (in terms of width in common fonts), and are not guaranteed to be unique. Think of what you might find on a credit-card-sized wallet or checkbook calendar, such as in English for days of the week:
S M T W T F S
typography
frenchspacing (yes or no) Enable or disable \frenchspacing
hyphenrules As named in language.dat.
lefthyphenmin \lefthyphenmin
righthyphenmin \righthyphenmin
hyphenchar The hyphenation char (number). Empty for the default. 0 if there is no hyphen (eg, Thai).
prehyphenchar Not yet used (luatex).
posthyphenchar Not yet used (luatex).
exhyphenchar Not yet used (luatex)
preexhyphenchar Not yet used (luatex)
postexhyphenchar Not yet used (luatex)
hyphenationmin Not yet used (luatex), but it will be soon.
hyphenate.other.locale (Tentative syntax.) A few hyphenation
patterns require setting some chars to other. This one is based on
the language.
hyphenate.other.script (Tentative syntax.) Same, based on the script.
linebreaking The line breaking mode, with a letter: h(yphenated, by default), c(jk), s(outheast Asian), u(nhyphenated).
labels
Under development:
https://github.com/latex3/babel/blob/master/news-guides/news/whats-new-in-babel-3.48.md
characters
See the CLDR. For example Exemplar
Characters,
can help to recognize a language. This list and the punctuation list
are currently not used by babel.
numbers
See Numering systems
The section about numbers may be used by some package to format
numbers (or even babel itself in a future). They reflect local tradicional
usage, not the international one set by either the SI or ISO 80000.
counters
See https://tex.stackexchange.com/questions/529813/how-to-define-counters-with-arbitrary-alphabet/530491#530491