XmTextPosition(3) on Linux
The text 56f311746a14, as Debian 13 (Trixie), Debian 12 (Bookworm), Debian 11 (Bullseye), Debian 10 (Buster), Debian 9 (Stretch), Debian 8 (Jessie) shipped it.
Data type for a character position within a text string "XmTextPosition" "data types" "XmTextPosition"