If 'isAutoCollect' flag is True at the time of subscription creation, then the recurring payment collection runs at auto-pilot mode. It is automatically triggered with no manual intervention from your side. On the due date of the payment collection, as specified per the subscription contract during create subscription, Paytm would automatically process the payment deduction. Once the transaction is confirmed then a Payment S2S callback is provided on your configured callback URL by Paytm.
If 'isAutoCollect' flag is False at the time of subscription, then Renew Subscription API needs to be triggered with the subscription ID, provided at the time of subscription creation. After that, the customer gets debited and acknowledgment is shared with you.This payment collection request is to be sent by the merchant, and the customer needs no intervention in the payment collection process.
Note: for Variable Amount/ On-demand and Daily frequency subcriptions, It is mandatory to collect payments manually.