I'd like to see a wiring diagram for the board, there's no mention of current limiting resistors, so assume it has some. My guess though is thst active low means pulling the input to ground for current to flow through the opto's diode which is connected to the supply rail and active high means the diode is grounded and a voltage is applied to activate it..

Assuming active low, a 2-wire switch is wired from input to ground, + side to input. An NC switch is always preferred so the relay is always energized and loss of supply or wire break is a fault.

A 3-wire NPN NC switch would be wired to +24v and ground, output to the input.

Similarly on the BOB input, the relay NO connection (as this is closed coz the relay is energized) is used as input to the BOB so again a break in the wiring or relay board is a fault condition.

Whether you need any other bits depends on whether pullup and/or current limiting resistors are in place.

Personally if you have open-ended optoisolated inputs on the BOB (i.e. access to both ends of the diode) the use of an intermediate relay board reduces reliability and adds complexity and cost for no real benefit.