iROAS (incremental ROAS) measures the extra revenue your mail piece actually caused, above and beyond what those customers would have bought anyway. Regular ROAS credits the mail with every sale; iROAS strips out a baseline so you can see the true lift.
That baseline comes from a holdout group — a small, randomly selected slice of your target audience that is deliberately not mailed. Their conversion rate stands in for "what would have happened anyway," and iROAS is calculated by comparing your mailed group's results against that holdout baseline.
If no holdout group was configured for a specific order or campaign, there's no baseline to compare against — so no iROAS figure is calculated for it, and you shouldn't rely on one that seems to be showing regardless. Also, a small holdout group can make iROAS swing unpredictably (one big holdout sale can throw off the math), so treat iROAS from a thin holdout as directional rather than exact. If you're unsure whether a holdout was set up for a given order, check with your LettrLabs contact before making decisions based on that order's iROAS.