Substitute one word or phrase with a different word or phrase. Often used to expand/clarify abbreviations.
My favorite chemical element is (Al)[sub:"aluminum"],
but Al prefers (Mg)["magnesium"].
The element is (Al)["aluminum"].
The element is (Al)[sub:"aluminum"].
The element is Al.
<speak>
The element is <sub alias="aluminum">Al</sub>.
</speak>
<speak>
The element is <sub alias="aluminum">Al</sub>.
</speak>