If the client intends to encash their full pension pot and they have no other source of income then form P50Z is required.
If the client intends to encash their full pension pot and they do have another source of income then form P53Z is required.
If the client only intends to encash only some of their pension pot and they don't intend to take any further payments or income from it just now then form P55 is required.
If the client only intends to encash only some of their pension pot and they do intent to take any further payments or income from it HMRC will confirm the tax code to be used.