There are a number of applicable standards. I am no expert on them. Aside from local country specific standards like AS4024 here in Australia, International standards include ISO 13850, IEC 60204-1 and IEC 60947-5-5
on a home machine, often a 2 pole estop button is used. one pole cuts mains power to spindle and stepper motors and the other is low voltage circuit wired to linuxcnc to tell the controller estop has been pressed. But this does not safely bring heavy machines to a stop so it gets more complicated. Also its not helpful if you have the control panel at one end of a plasma table and you want to place an estop at the other end where the console is. One approach might be to run a pull rope right around the table but running mains power to another distant switch clearly adds its own safety risks.
I suspect estop-latch predates the standards but it does allow you to create an estop chain in software. It is somewhat complicated to understand initially and I find its best to draw it on paper as a series of boxes (one for each estop-latch) each of these boxes has an OK-out pin which needs to connect to the reset pin on the next in the chain with the last OK-out connected to iocontrol.emc-enable
Each latch will have either a fault-in or ok-in pin attached to the estop button or door latch or whatever that needs to be safe before the machine will start. Finally, the iocontrol.user-request-enable pin is attached to the reset pin on the very first latch. So when the user requests the machine to be enabled (usually the orange button in axis), a short pulse is sent to the first reset and if all faults are clear, the sigal to start the machine arrives at linuxcnc. The beauty of this approach is that you can just as easily use a momentary push button to act as that reset signal if you wish.
Taking that into the standards says the controller can have no part in the safety circuit, and every sub-circuit and every contactor needs to have dual redundancy so a fault will not trigger anything and both circuits need to work before something's enabled. Also if there is a power outage, the machine cannot become live again when power is restored. You need to press that reset button. Also every contactor and every relay needs to be a force guided one where a non conducting pin is forced between relay contacts to guard against them welding closed from an arc.. Aside from that the safety circuit or safety relay does the same thing as estop-latch. It won't enable the machine until every component being monitored is safe.
