/text/scalc/01/05060000.xhp see spec "Merge cells function changed to toggle"

Merge Cells Combines the selected cells into a single cell or splits merged cells.

The merged cell receives the name of the first cell of the original cell range. Merged cells cannot be merged a second time with other cells. The range must form a rectangle, multiple selection is not supported. If the cells to be merged have any contents, a security dialog is shown.