Use an expression when one dynamic token is only part of the required value. concat joins text fragments in order. split returns an array of pieces using the delimiter you choose, and first selects the first piece. An expression entered as a whole value can return text, a number, an array or an object. Interpolation inside surrounding text produces a string.
Read the source format before splitting. A reference such as NW|714 has a different delimiter from NW-714, and an empty first segment stays empty. The exercises use fictional dispatch labels so you can inspect the exact result. Put a Compose action between the trigger and the email when you want a named intermediate result in Run history. This makes a wrong delimiter easier to find than hiding the whole transformation inside an email field.