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As noted in the comment, it looks like this property (currently?) only supports one item.  It is not clear why it is plural - it may be that in the future it is expected to expand to accept a `string[]` as well?

I'm not familiar enough with FSx, but I do have hosts spread across multiple subnets. Can I give any subnet in Pulumi and then reconfigure it via AWS console later?

From <https://aws.amazon.com/fsx/windows/faqs/>:

&gt; Q: How do I deploy multi-AZ file systems with Amazon FSx?

&gt; A: For workloads that require multi-AZ redundancy to tolerate temporary AZ unavailability, *you can create multiple ﬁle systems in separate AZs*, keep them in sync, and conﬁgure failover between them, using Amazon FSx for Windows File Server’s support for Microsoft’s Distributed File System (DFS) Replication and Namespace features. DFS Replication allows you to automatically replicate data between two file systems, and DFS Namespace allows you to configure automatic failover.

Though to your specific question - you should have no problem reaching the FileSystem from hosts in other AZs - you'll just have a slight SPOF AZ issue because it appears FSx is a zonal not regional service.