In this methodological note we discuss several topics related to interpretation of some basic cosmological principles. We demonstrate that one of the key points is the usage of synchronous reference frames. The Friedmann-Robertson-Walker one is the m
ost known example of them. We describe how different quantities behave in this frame. Special attention is paid to potentially observable parameters. We discuss different variants for choosing measures of velocity and acceleration representing the Hubble flow, and present illustrative calculations of apparent acceleration in flat $Lambda CDM$ model for various epochs. We generalize description of the tethered galaxies problem for different velocity measures and equations of state, and illustrate time behavior of velocities and redshifts in the $Lambda CDM$ model.
