Context
When I try to create a new Compute resource and picking it in Azure portal Marketplace the description starts with "Bring Your Own License enabled" I suppose the Compute resource will be a Windows Server with IIS configured and a web application installed.
Question
What does "Bring Your Own License enabled" in this context exactly mean?
Bottom line: If I would like to use this resource do I have to own an MS license a) or not b)?
There are two ways to license software on hardware provisioned through the Azure marketplace.
The first is an "embedded" license. This means that when you start up a templated compute resource (in your case a VM with Syncfusion), you don't need to buy an additional license for Syncfusion. You will notice that the usage cost for the VM is inflated compared to a vanilla VM without the software installed on it. The cost of the license is baked into the price you pay to "rent" the VM from Microsoft via Azure subscription.
The second is "Bring your own license". This is typically used when an organisation has existing software licenses already purchased (in this case, for Syncfusion). When you bring your own license, you only pay for the vanilla virtual machine and apply your pre-bough Syncfusion license. In other words, the cost of the syncfusion license is not built in to the rental of the virtual machine.
This can get more complex when you factor in templates with multiple software packages installed but hopefully you get the drift.