Many modern communication systems use modulation types that combine amplitude and phase manipulations. For a better understanding the pattern diagrams are used. If you draw dividing lines between the spots representing "perfect" aplitude and phase positions for 0 and 1 levels, you can see how any uncertainty in amplitude and phase due to noise and jitter can degrade the modulation/demodulation structure.
Depending on position (spot) numbers we have QAM, PSK, QPSK and e.g. 256 PSK modulation systems. More spots offer a higher vulnerability by noise and interference. In the complete system all this affects the Bit Error Rate. Additional error-correction coding offers an improvement in BER for a cost of a lower transmission capacity (more bits must be transmitted for corrections than for data transmission itself).
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To add: if the spots are arranged on a circle it means a constant signal amplitude, so the modulation is a PSK type. Squares and similar structures indicate that also amplitude is manipulated, so QAM.
Please go and read some basics like books by Feher.