50 ohms. Do not terminate.

In the Fury manual the CMOS and 1PPS Out the descriptions of the outputs state "50Ohms, Do Not Terminate." What is the meaning of this?

The output is designed for a reflected-wave 50 Ohms open-ended transmission line/coax system. The reason for that is lower power consumption, and lower to no-modulation of the internal rails with 1Hz and harmonic noise. See here for a description of how that works: https://en.wikipedia.org/wiki/Reflected-wave_switching It assumes a 50 Ohms transmission line such as a coax cable, that does not have a 50 Ohms resistive end-termination. So, a simple RG-316 coax cable with the customer equipment having an input termination of 1K Ohms or higher is what this is designed for.