Hi, Phil
Some control boards/devices require a high setting to be pulled low thy float at 5 volts and when pulled low with Mach cause a step, Some a low and when
pushed high thy step. Some control boards/devices won't work at all in the wrong Hi/Lo state.
In other devices let's say a Small 5 volt Relay, If one end is tied to 5 volts, The other end needs to be pulled low to close it, Gnd one end, Other end
needs to be pushed high, Thus Active Low, Is ether "X" or "Checked".
Hope this Helps, Chip