Where can I find the documentation on the key "speller/initials"?

[rodrigomorales1]

I'm not a Chinese native speaker, but I'm learning Chinese and I like using RIME.

I want to create my own custom schema, so I'm looking at some schemas that the community have created so that I can better understand how RIME schemas are created.

I have noticed that many schemas define the key initials. In bopomofo.schema.yaml,initials is set as shown below (link):

(...omitted lines...)
speller:
  alphabet: '1qaz2wsxedcrfv5tgbyhnujm8ik,9ol.0p;/- 6347'
  initials: '1qaz2wsxedcrfv5tgbyhnujm8ik,9ol.0p;/-'
  finals: " 6347"
  delimiter: "'"
(...omitted lines...)

In double_pinyin_mspy.schema.yaml , initials is set as shown below (link):

(...omitted lines...)
speller:
  alphabet: zyxwvutsrqponmlkjihgfedcba;
  initials: zyxwvutsrqponmlkjihgfedcba
  delimiter: " '"
  algebra:
(...omitted lines...)

So, my question is: Where can I find the documentation on the key "initials"?

I know of the existence of https://github.com/rime/home/wiki/RimeWithSchemata , but that page doesn't contain documentation on the key initials.