This one’s easy: all the ones you’re not using. Well, that’s not completely true but it’s close. The port labeled “PWR” doesn’t need a terminator, but you should really be using that for your power inserter. The legacy ports don’t need terminators as long as they’re covered in dust caps, but if you’ve lost the dust caps you can use terminators with no problem. But basically, if you see an open coax connector, put a terminator on it.
The SWM-16, just like any device that uses coaxial connections, will put signal out through every open port unless it specifically knows not to. It would have been possible to make the SWM-16 self-terminating but terminators are so cheap that there doesn’t seem to be a need for the extra engineering.
And while I’m on my soapbox, take a few minutes and put terminators on every open coax connector in your home. I mean, they’re cheap to buy, and at the very least they will keep the connections clean and free of corrosion. They certainly won’t hurt.