See Annuity is basically a "Series of Cash Flows" which
can be either Inflows or Outflows depending upon your
Question.
So "Residual Value of Annuity" tells about the
LAST CASHFLOW (ie
Inflow or Outflow depending upon your Question) of the Series of
Cashflows that we talked in above point.
Lets Understand with an Example: if Mr X has an Annuity
Plan for his Pension Fund, it is interpreted as he has a series of
regular Cash Outflow which he has to Deposit till "n" period. And
if we say that "residual value of Annuity is 30% of the Plan
Amount" it would mean that last Annuity Payment would be 30% of the
Total Plan Amount.
So in your case it says that Last Annuity Cashflow
(receivable or payable as per your ques) would be 30% of the
Purchase Cost.