Imagine that the three mutations of gene P are nonsense mutations; there are three unusual stop codons like UAG (amber codon), UAA (ochre codon) and UGA (opal codon) that are responsible for the suppression.
For instance, the amber suppressors will suppress only the mutants of amber but not ochre or opal in this way a haploid nonsense suppressor su1 acts on mutation 1, but not on mutation 2 or 3 and in an unlinked nonsense suppressor su2 works on mutation of 2, but not on 1 or 3 of the gene P.