Dear Student,
Outstanding Subscription is a current asset for a Not for Profit Organisation.
Following is the treatment of outstanding Subscription:
1) Outstanding Subscription in the beginning of the year ( say as on 1 April 2018) = To be recorded on the debit side of Subscription Account as 'To Balance b/d'.
2) Outstanding Subscription at the end of the year ( say as on 31 March 2019 ) = To be recorded on the credit side of Subscription Account as 'By Balance c/d'.
Further, the outstanding subscription at the end of year is a part of the current year's subscription amount and it is added to the income of current year as well in the Income and Expenditure Account of the NPO. So closing outstanding subscription is an income of the current year.
Regards