For Apple (iOS) devices only, customers now have the ability to purchase a one-off In-App Purchase using their Apple ID account. This is not a monthly or annual subscription but a one-off purchase price.
This means that customers will have the ability to view a single model purchase for an unlimited length of time.
Unlike monthly subscribers, however, In-App Purchase customers (Apple iOS only) will need to make an additional purchase for additional variations (e.g. Hi-Res or Multi-storey).
See the article In App Purchase vs Monthly Subscription.