ICD-10 Codes D47z.1 and D47z.9

I have found no explanation of z as a place holder, if that is what the intent is on this. There is nothing in the official guidleines on this either. What is the significane of z in D47z.1 and D47.9? If this is supposed to be a place holder why use z when we alkready have place holder x? Can you please refer this question to Nancy McQuire? Thanks.
Pat Strubberg, CPC, CPMA, CPC-I
Coding Manager
Patients First Health Care
901 Patients First Drive
Washington, MO 63090
636-390-1743
patstrubberg@pateintsfirsthc.com

re: ICD-10 Codes D47z.1 and D47z.9

Post-transplant lymphoproliferative disorder (PTLD)
D47.Z1 is a billable ICD-10-CM code that can be used to specify a diagnosis. there is a note that states Code first complications of transplanted organs and tissues (T86.-)
This is not a place-holder it is the subcategory letter for the condition code.
D47.Z1, Post-transplant lymphoproliferative disorder (PTLD)
D47.Z9, Other specified neoplasms of uncertain behavior of lymphoid, hematopoietic and related .
You'll also see an alpha used for Merkel cell carcinoma (ex, category C4a.-)