If you are labeling your products for FBA, you must be sure to follow Amazon's FBA Barcode Guidelines. According to Seller Central:
All Amazon barcodes must be printed in black ink on white, non-reflective labels with removable adhesive. Dimensions must be between 1 x 2 inches and 2 x 3 inches (1 x 3 inches or 2 x 2 inches, for example).
The actual label, itself, must contain the following:
Sufficient white space around the Amazon barcode and related text (0.25 inches on the sides and 0.125 inches on the top and bottom)
ASIN or FNSKU
Product name
Item condition
In terms of placement, you must make sure of the following:
Put the correct barcode on each item.
Cover all other visible barcodes, except for serial number barcodes or Transparency authentication code labels.
Place the barcode on the outside of any prep materials.
Do not place the barcode on a curve or corner of the package.
Allow 0.25 inches between the edge of the label and the edge of the packaging.
Ensure that each item in a case pack has an Amazon barcode and that any barcodes on the case have been removed.
For more information, see the article Use an Amazon barcode to track inventory.