An Offer is the template of a Subscription.

When a Customer subscribes to an Offer, it makes a Subscription.

The Subscription is related to a Customer and an Offer.

You can modify the Offer, it will not modify the Subscription of your Customer.
You can modify the Subscription of a Customer, it will not affect the Offer related to this Subscription.

The Subscription is a copy of the Offer made when the Customer subscribed.

See ‘ProAbono – Core Concepts’.