Constant OFF_STATE 

pub const OFF_STATE: &str = "Off";

The off state’s appearance name, which every check box and radio button shares.

A file may name the on state anything; this is the only reserved spelling.

use pdfrum_form::field::{ToggleState, toggle::OFF_STATE};

let mut state = ToggleState::new(OFF_STATE, "Yes");
state.set_checked(true);
assert_eq!(state.state, "Yes");