A SKU (Stock Keeping Unit) is a string of characters used to identify a distinct item. A SKU must be:
- composed of letters, numbers, or hyphens
- 25 characters or less
- unique among all other items in your company
A unique SKU code is required for:
- every possible permutation of an item;
- every possible permutation of bundled items -- e.g. a SKU sold in an online store that contains 3 other distinct SKUs;
- every base component of an item that each will be received, stocked, and tracked in inventory, then later assembled into a different, distinct finished-goods item, which will also require a unique SKU code;
- custom packaging.
Every product variation requires its own SKU code. Attributes which distinguish items and necessitate unique SKU codes include and aren't limited to color, design, size, style, and even production method and whether or not an item is signed.
- A single screen-printed t-shirt design available in one color in both men's and women's styles with five sizes for each style would require 10 SKUs.
- If a book is available signed and unsigned, two SKUs would be required.
- A single design available in both white and black mugs in both 11-ounce and 15-ounce sizes would require four SKUs.
Regarding the production method, a given t-shirt that will be screen printed requires a unique SKU, and the exact same design to be printed on the exact same blank t-shirt also requires a distinct SKU code if the printing will be via Direct-to-Garment instead of Screen Print.
For more info, see "How can I format my SKUs?"