Stacked text is the term used in AutoCAD for fractions, superscripts and tolerances etc. Depending on what kind of stacked text is required it is done differently as outlined below. Common for all stacking types, is that they are only available in Multi Line Text (MTEXT), and not in Single Line Text (TEXT, DTEXT). It is possible, however, to insert symbols in Single Line Text, please see Adding symbols to text.
Superscript Text
In the MTEXT Text Formatting window, follow the desired text to be stacked with a “^” (e.g. AB^), highlight the text, right click and select Stack from the menu selection.

Subscript Text
In the MTEXT Text Formatting window, precede the desired text to be stacked with a “^” (e.g. ^AB), highlight the text, right click and select Stack from the menu selection.

Diagonally Stacked Text
In the MTEXT Text Formatting window, separate the desired text to be stacked with a ”#” (e.g. AB#CD), highlight the text, right click and select Stack from the menu selection.

Vertically Stacked Text with Line Separator
In the MTEXT Text Formatting window, separate the desired text to be stacked with a “/” (e.g. AB/CD), highlight the text, right click and select Stack from the menu selection.

Vertically Stacked Text without Line Separator
In the MTEXT Text Formatting window, separate the desired text to be stacked with a ”^” (e.g. AB^CD), highlight the text, right click and select Stack from the menu selection.

An alternative option to right clicking and selecting Stack from the menu selection is to highlight the text and click on the Stack button on the Text Formatting bar.

Stacked text can be customized by highlighting the stacked text (from within the Text Formatting window), right click and select Properties from the menu selection.

To add symbols to text, please refer to this FAQ: Adding Symbols