What format should a garment label use?
UPC-A or EAN-13 for the retail-facing size tag, the same as any other retail product. The barcode identifies the product (and often the size and color as separate GTINs, not variants of one code), so it needs a registered number the same way any retail item does. See do you need a GS1 number.
Why does orientation matter more on a garment tag?
Because folded or hanging tags often get scanned at an angle, and a barcode is more tolerant of being scanned across its bars than along them. Print the barcode running perpendicular to the fold line where possible, so a natural fold doesn’t land across the middle of the symbol and obscure a bar.
Does the tag material change anything?
Only in that it needs to hold print quality after handling. A barcode that’s crisp when printed but smudges, stretches, or fades after the tag passes through a few hands fails the same way a badly printed one does. Test a sample tag after it’s been through normal handling, not just fresh off the printer.