What does pg_stat_get_xact_function_self_time count exactly?

Hi,

Is the self-time of a tracked function (as reported by
pg_stat_get_xact_function_self_time) its total time minus

- the time of other tracked functions it calls?

- the time of other tracked or untracked functions it calls?

- the time of other tracked/untracked functions or SPI functions it calls?

- something else?

Regards,
-Chap