An ArcGIS subscription is a subscription plan that gives a developer, group of users, or an organization access to ArcGIS Platform location services and ArcGIS Developer website, APIs, tools, applications, content, and services.
There are two subscription types:
1) ArcGIS Online subscription: for an organization or a group with multiple users with different access requirements that wish to share and collaborate with members inside or outside of their organization. ArcGIS Online subscriptions consume credits for access to credit-consuming location services. Additional credits can be acquired as needed by the organization administrator.
2) ArcGIS Developer subscription: for a single user or app developer using client APIs and app templates to access location services. Developer subscriptions utilize a free tier of location services and then activate pay-as-you-go and monthly billing for the services consumed by their apps.
Both subscription methods provide access to ArcGIS Developer and ArcGIS Online websites and tools.