The optos are normally open. so when the Reset input is powered, it pulls the relays in, as long as the power feeds from whatever they monitor are in the correct states.
Each powered relay then latches itself, as long as its safety input stays at the correct voltage.
Think of the opto as the "on" button in a simple latching relay circuit, with whatever safety device controlling power as the "off" button; almost like this, except no B2 and "off" is by removing power to the whole circuit.