Reads Mapped : R2 reads that successfully mapped to linker and tag-barcode.
Reads Unmapped too Short : Unmapped R2 reads because read length < linker length + tag-barcode length.
Reads Unmapped Invalid Linker : Unmapped R2 reads because of too many mismatches in linker sequence.
Reads Unmapped Invalid Barcode : Unmapped R2 reads because of too many mismatches in tag-barcode sequence.