Seed match segments have Watson-Crick (WC) complementarity ("matching") to positions 2-7 ("seed") of a miRNA/siRNA. We use the notation m1, m2,... to refer to miRNA/siRNA bases, starting at the 5'-most base, and t1,t2,... to refer to positions in the target mRNAs opposite miRNA /siRNA base m1, m2,..., respectively.
The different seed match types are illustrated for miR-1 above. A1 indicates presence of an adenosine opposite miRNA/siRNA base 1 (independent of nucleotide composition at miRNA/siRNA base 1). M8 indicates a WC match to miRNA/siRNA base 8.