Meaning of plus symbol "+" in bcl2fastq reported barcodes

Question

[MutationalMeltdown]

bcl2fastq v.2.20 is reporting this...

Barcode collision for barcodes: XXXXXXXX+XXXXXXXX, XXXXXXXX+XXXXXXXX

(ATCG sequences here not Xs)

I can prevent the error by changing the number of barcode mismatches (currently set at 2), but my question is what does the plus symbol "+" separator represent here? Thanks

Answer 1

[MutationalMeltdown]

Dual indexes.........