A field to return the value of a different field

Hello,

I want to create a field that returns/mirrors that of another field at different "stages" in our sales cycle. The field would be essentially copying the value from a text-multiple choice field when a certain stage (text -multiple choice) field is met. 

What type of field would this be? Would it also be an automation?

Thank you!
Jenna

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Jenna Odom
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