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I wasn't there two decades ago when the spec was first being developed, but based on other parts of the spec they're clearly attempting to establish a parallel with [`/usr/local`](https://refspecs.linuxfoundation.org/FHS_3.0/fhs/ch04s09.html). The intended minimal `PATH=~/.local/bin:/usr/local/bin:/usr/bin:/bin`; `XDG_DATA_HOME` and `XDG_DATA_DIRS` form a similar order for data files that, were it to be represented in a single environment variable like `PATH`, would look like `~/.local/share:/usr/local/share/:/usr/share/`.

Whether this was the XDG's idea or whether they were codifying an existing informal convention, I don't know. The use of `~/.config` as the default for `XDG_CONFIG_HOME` instead of `~/.local/etc` suggests the latter.