private Integer getRelativeRowIndex() {
        Integer relativeRowIndex = relativeRowIndexMap.get(currentUniqueDataFlag);
        if (relativeRowIndex == null) {
            relativeRowIndex = 0;
        } else {
            relativeRowIndex++;
        }
        relativeRowIndexMap.put(currentUniqueDataFlag, relativeRowIndex);
        return relativeRowIndex;
    }
The unique data encoding for this fill
fastexcel-core/src/main/java/cn/idev/excel/write/executor/ExcelWriteFillExecutor.java