key

$ git splice key -h | check-help preamble
NAME
    git-splice-key - Print or rename an upstream's key

SYNOPSIS
    git splice key [--upstream <name>] <path> [<new-key>]

Description

$ git splice key -h | check-help section description
    Prints the key splice <path>'s upstream has in this repository: its
    fetched branches are refs/splices/<key>/heads/<branch>, e.g. for
    'git log splices/<key>/heads/main'.

In the walkthrough's repositories, vendor/pkg-a/ has already been fetched, so its upstream has the key pkg-a:

$ git splice key vendor/pkg-a
pkg-a

Options

$ git splice key -h | check-help section options
    --upstream <name>
        Names the upstream, as <path>/.splice does in [upstream "<name>"].
        Without it, the splice's default upstream.

<new-key>

$ git splice key -h | check-help option new-key
        Renames the key and moves its refs along. Renaming it also renames
        it in the tracked upstream branches, branch.<branch>.splice =
        <key>:<name>.

Rename it to a:

$ git splice key vendor/pkg-a a
ok   vendor/pkg-a: renamed key 'pkg-a' to 'a'

The fetched branch moves with it:

$ git log --oneline -1 splices/a/heads/main
703b936 pkg-a: a second commit, after the clone

Keys

$ git splice key -h | check-help section keys
    A key belongs to an upstream URL, so splices with the same URL share it.
    The first fetch of an upstream records its key in the repository's
    config, as splice.<key>.url, from the end of its URL: lib for
    https://github.com/x/lib.git, or x-lib, then github.com-x-lib, if that's
    taken. A key is lower-case letters, digits, ".", "_" and "-", at most 64
    long.

A key belongs to an upstream URL, so every splice of that URL shares it (URLs, keys and refs, choosing a key). How the commands change these refs, and what they cost: splice refs.