Symbol LS 6000 Series Product Reference Manual page 85

UPC/EAN Security Level 1
As bar code quality levels diminish, certain characters become
prone to misdecodes before others (i.e., 1, 2, 7, 8). If you are
experiencing mis-decodes of poorly printed bar codes, and the
mis-decodes are limited to these characters, select this security
level.
UPC/EAN Security Level 2
If you are experiencing misdecodes of poorly printed bar codes,
and the misdecodes are not limited to characters 1, 2, 7, and 8,
select this security level.
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LS 6000 Series Product Reference Guide
UPC/EAN SECURITY LEVEL 1
UPC/EAN SECURITY LEVEL 2