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1#2 = Text stacked diagonally with a line separator.
1/2 = Text stacked horizontally with a line separator
1^2 = Text stacked horizontally without a line separator (aka "Tolerance")
3^ = Superscripted text (eg. 3.0m3)
^2 = Subscripted text (eh. H2O)
Once you enter the code, simply highlight the characters to be changed, and then click the A/B button.
For even more control, right-click on the stacked text (while it's highlighted within the Mtext editor) and select Properties from the shortcut menu to open the Stack Properties dialog box.