A batch identifier
A dye lot identifies a manufacturing batch of a yarn color. Labels with the same color name can come from different lots, and those lots may look different in a finished project.
Plan before buying
Check the lot numbers when buying multiple balls. Obtain enough for the project, including any chosen extra for swatching or changes. Compare yarn in good light if matching an existing ball. A matching lot number helps, but still inspect the yarn.
Use the yarn substitution calculator to compare lengths before purchasing. It does not predict color differences or guarantee availability.