Speaks the value as a unit. Can be a number and unit or just a unit.
Units values include:
Some implementations of SSML converts units to singular or plural depending on the number.
I would walk (500 mi)[unit]
n/a
I would walk (500 mi)[unit]
I would walk 500 mi
<speak>
I would walk <say-as interpret-as="unit">500 mi</say-as>
</speak>
<speak>
I would walk <say-as interpret-as="unit">500 mi</say-as>
</speak>