Columns are, from left to right:

*primer_set_id*   (rows go by pairs, e.g.: "AF426268_chr21_41730852_41730931" is the external (first) set of primers, "AF426268_chr21_41730852_41730931_internal" corresponds to the internal (second) one)
*fwd_primer_seq*   (5' to 3')
*rev_primer*   (5' to 3')
*target_seq*   (5' to 3', predicted transcript sequence or subsequence)