Yes, the 2703 is designed for stand-alone operation or as a "master". I've only used it stand-alone since I don't own a 2705. The 2705 can only be "slave" to a 2703. The 2703 will produce a single phase, and you can optionally add one or two 2705s to gain additional signals with the same frequency but a phase shift relative to the 2703. You might use this to simulate a three phase voltage source, for example. Valhalla also made three phase power meters. So I'd say the 2705 is more much niche than the 2703 unless it could be made to work standalone. The 2703 can not be a "slave" to another 2703. It misses the "to master" cable connector. See
here. On page 8-2 of the manual is a diagram of how a 2703 and one or two 2705 are to be connected.
The 2703 generates a clock signal at the desired frequency, and the 2705 follows this frequency but introduces a phase shift. So it might be 120 degrees behind the 2703. It's described in the theory of operations in the 2705 part of the manual.