Perhaps you mean 4B5B? That's a way to assign combinations of 5 bits to every combination of 4 bits, in such a way that you avoid transmitting long strings of only zeros or only ones. This avoids synchronization problems.
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A mapping is a relationship between two sets. Given sets A and B (which need not be different) a mapping allocates an element of B to each element of A.
A relation is a mapping or pairing of input values with output values.
It is simply a mapping. It could be a function but there are several conditions that need to be met before the mapping can become a function and there is no basis for assuming that those conditions are met.
3d mapping
A function is a relation whose mapping is a bijection.