You can copy the value form some source field to some destination field. Be aware that source and destination field must both have the same data type. For example you can copy a checkbox custom field to another checkbox custom field, but you cannot copy a text date custom field to a checkbox user picker custom field.
If the destination field is some kind of a text field (for example Summary, Description, Environment, Free Text Field, ...) it often also works even the source field has a different data type. But you should test this first.
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