BillingTemplate generates Purchase objects, either to issue one-time invoices or in a subscription.
It does so by copying over its' PurchaseDetails, one of its BillingTemplateClient-s and generating other fields from BillingTemplate's fields as necessary into a new Purchase object.
If is_subscription is true, it is considered to be a subscription's BillingTemplate. You will need to specify subscription_* fields like subscription_period when creating it and add BillingTemplateClient objects to its billing cycle (POST /billing_templates/{id}/add_subscriber/). After that the clients will receive recurring invoices (that will be paid for automatically if client saves their card) according to the BillingTemplate settings you have specified.
If is_subscription is false, this BillingTemplate is used to send one-time invoices. After creating it and specifying invoice_* fields, use POST /billing_templates/{id}/send_invoice/ request to send the actual invoices. BillingTemplateClients for non-subscription BillingTemplates are not saved.