If you use both internal/external drives, you should leave it empty.
You terminate at the end of the physical scsi chain.
That said, there are 2 different terminators used for SCSI devices, one typ is used in pairs, one in 3s. TT use 3 pcs obviously.
I have both kinds lying around.
That said, if you use only one drive, termination is not always necessary. As long as the drive is terminated, it usually works if no other units are present.
So I suspect that getting those terminators would not be absolutely necessary.