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"amihomo" has described soft decoding, where a decision on a recived symbol is not taken immediately after receiving one symbol, but a whole sequence is examined before a decision is taken (viterbi decoding being the prime example).
However, I guess what the OP is asking about is soft demapper. The demapper associates the recieved signal to a constellation point. In hard decoding, it choses the nearest valid constellation point and decalres that as the recieved symbol. When soft decoding is done, the demapper gives out a set of values (each being a few bits), each being the quantised distance of the recieved signal from all, or a subset of, valid constellation points. The ECC decoder that follows the demapper usually gives a better performance if soft info is used.
So soft decoding can be done with both soft and hard demapping.