Azure Subscriptions are containers that hold your Resource Groups (RGs) and with that your Resources. It might make sense to have multiple subcriptions for separation of billing or management.

• You can have multiple Subscriptions trust the same Azure Active Directory (AAD)
• Each Subscription can only trust a single directory
• Billing is done per subscription
• Resources can be moved from one Subscription to another if both Subscriptions are in the same AAD Tenant