What do you mean "it is suggested" ?
A repetition of 1x1x1 for the DynamicalMatrix calculation means no repetition at all. There should never be any repetition along non-periodic directions. This means that:
a) For 1D devices like nanotubes and nanoribbons, the repetition should be 1x1x1.
b) For 2D devices, like nanosheets that are periodic along B, the repetition should be 1 x N x 1, where N = 3 or 5 or 7 or 9, etc.
c) For 3D devices, which are periodic along both A and B, the repetition should be N x M x 1.
Note that the "automatic" repetition option sometimes selects repetition > 1 along non-periodic directions. This happens when the distance to the supercell side along that direction is relatively small. In that case. Manually set the repetition. That is the correct approach, and also saves considerable computational time.
Finally, for phonon calculations on bulk configurations (not devices) the repetition should usually be non-zero along all directions: N x M x K.
