Pin 14 is one of the buffered outputs, no different from the others except that it is used to control more than one device. Here is a description of the BOB that might be informative.
Pins 10, 11, 12, 13 and 15 are the inputs for the limits switches or whatever you chose to connect to them. The switches are powered by a 10 volt supply regulated down from the 24 volt supply. Their connection to the parallel port inputs is isolated through 5 of the opto couplers.
The 6th opto coupler connects pin 1, an output from the parallel port, to the circuit that changes the PWM spindle signal to analog. In the BOB diagram just to the left of the 6th opto-isolator there are two triangle symbols. These the sections of the SN74HD245s that buffer pin 1. Those sections and the 10K resistor are also shown on the left side of the diagram as IC2 and the resistor attached to pin 1.
Other outputs from the parallel port are not opto-isolated by the BOB. They are buffered by the SN47HC245 ICs and connect to the terminal strip. Stepper signals also connect to the 4 pin connectors. The opto couplers in the drivers isolate them from the parallel port and the 5V supply.
Pin 17 can be jumpered to operate the relay. That circuit is on the middle right of the diagram. The resistor and SN74HC245 section are also shown on the left like the analog circuit. The relay common is shown connected to the 24 volt common. That seems a bit strange as relays are often used to isolate circuits.
The relay, the input switches and the analog output share the same common which is isolated from the parallel port, the buffered outputs and 5 volt common. The parallel port receives ground from the PC. The 24 volt common should be separately grounded for safety, especially if the connections to the limits switches are not protected.