$XDG_CONFIG_HOME is clearly meant for data that would otherwise be in a configuration style dotfile. This is the sort of configuration that that users may well want to sync between machines such that a dotfile manager should include them by default. (That said, there may be some data from some of the other XDG directories worth syncing too, but generally a dotfile manager should not include those other folders in full by default.)
Beyond that the spec defines $XDG_DATA_HOME as "base directory relative to which user-specific data files should be stored.". The default path ($HOME/.local/share) might suggest that this should be read-only data, but it actually is read-write.
The description of the $XDG_STATE_HOME makes it quite clear that $XDG_DATA_HOME read-write: "The $XDG_STATE_HOME contains state data that should persist between (application) restarts, but that is not important or portable enough to the user that it should be stored in $XDG_DATA_HOME". This makes it clear that important and portable data should be written to $XDG_DATA_HOME.
And thus we also have $XDG_STATE_HOME. The list of examples that follow the previously quoted sentence make it clear that this is stuff that gets persisted, but if it were omitted from a backup, the user will probably not care too much (i.e. stuff like logs, history, set of open files, undo history, etc.
$CDG_DATA_HOME does also include any read-only reference data, which should be searched for there, and in $XDG_DATA_DIRS (which defaults to "/usr/local/share/:/usr/share/").
It could certainly be argued that read-only reference data and read-write user data should not have been mixed. After all Unix does keep them separate that at the top level `/var` vs `/usr/share`, but the spec authors apparent felt this distinction was not necessary at the user level.
$XDG_CONFIG_HOME is clearly meant for data that would otherwise be in a configuration style dotfile. This is the sort of configuration that that users may well want to sync between machines such that a dotfile manager should include them by default. (That said, there may be some data from some of the other XDG directories worth syncing too, but generally a dotfile manager should not include those other folders in full by default.)
Beyond that the spec defines $XDG_DATA_HOME as "base directory relative to which user-specific data files should be stored.". The default path ($HOME/.local/share) might suggest that this should be read-only data, but it actually is read-write.
The description of the $XDG_STATE_HOME makes it quite clear that $XDG_DATA_HOME read-write: "The $XDG_STATE_HOME contains state data that should persist between (application) restarts, but that is not important or portable enough to the user that it should be stored in $XDG_DATA_HOME". This makes it clear that important and portable data should be written to $XDG_DATA_HOME.
And thus we also have $XDG_STATE_HOME. The list of examples that follow the previously quoted sentence make it clear that this is stuff that gets persisted, but if it were omitted from a backup, the user will probably not care too much (i.e. stuff like logs, history, set of open files, undo history, etc.
$CDG_DATA_HOME does also include any read-only reference data, which should be searched for there, and in $XDG_DATA_DIRS (which defaults to "/usr/local/share/:/usr/share/").
It could certainly be argued that read-only reference data and read-write user data should not have been mixed. After all Unix does keep them separate that at the top level `/var` vs `/usr/share`, but the spec authors apparent felt this distinction was not necessary at the user level.