On Azure, there are usually two order types: bring your own license (BYOL) and pay as you go (PAYG).
BYOL offers perpetual (normal series and v-series) and annual subscription (s-series) licensing as opposed to PAYG, which is an hourly subscription available with marketplace-listed products. BYOL licenses are available for purchase from resellers or your distributors, and the publicly available price list, which Fortinet updates quarterly, lists prices. BYOL licensing provides the same ordering practice across all private and public clouds, no matter what the platform is. You must activate a license for the first time you access the instance from the GUI or CLI before you can start using various features.
With a PAYG subscription, the FortiGate-VM becomes available for use immediately after you create the instance. The marketplace product page mentions term-based prices (hourly or annual).
In BYOL and PAYG, Azure charges separately for resource consumption on computing instances, storage, and so on, without the use of software running on top of it (in this case FortiGate).
For BYOL, you typically order a combination of products and services, including support entitlement. S-series SKUs contain the VM base and service bundle entitlements for easier ordering. PAYG includes support, for which you must contact Fortinet Support with your customer information. See Plans.
To purchase PAYG, all you need to do is subscribe to the product on the marketplace. However, you must contact Fortinet Support with your customer information to obtain support entitlement. See Creating a support account.
PAYG FortiGate-VM instances do not support the use of virtual domains (VDOMs). If you plan to use VDOMs, deploy BYOL instances instead.
PAYG and BYOL licensing and payment models are not interchangeable. For example, once you spin up a FortiGate-VM PAYG instance, you cannot inject a BYOL license on the same VM. Likewise, you cannot convert a FortiGate-VM BYOL instance to PAYG.