Short definition
The production run a batch of tiles came from. Tiles from different lots vary slightly in shade and in calibre (actual fired size), so a job should be ordered from one batch.
A dye lot — also called a batch or shade lot — is the production run a set of tiles was fired in. Ceramic and porcelain tiles take their colour from mineral glazes fired at high temperature, and no two runs come out identically. The variation is small, often invisible in a single tile held on its own, and obvious across a floor when tiles from two lots sit side by side.
Calibre is the second half of the same problem. Fired clay shrinks, and how much it shrinks varies between runs, so the actual size of a nominally 600 × 600 mm tile drifts by a millimetre or two between batches. Manufacturers sort the output into calibre groups and print the group on the box alongside the shade code. Mixing calibres in one floor shows up as uneven grout joints, because the joint absorbs the difference.
The practical consequence is an ordering rule rather than a laying rule: work out the full quantity for the job, including wastage, and order it all at once from a single batch. A second order placed a week later to cover a shortfall will very likely arrive from a different lot, and on a continuous floor there is nowhere to hide the change. Keep the spare tiles from the original order for future repairs, because they are the only tiles that will ever match.
Where a job genuinely needs more than one lot — a large commercial floor that exceeds a single run — the usual approach is to open several boxes at once and blend tiles across the floor as they are laid, so the transition is gradual rather than a visible line. That is a decision to make before laying starts, not a fix afterwards.