Arnold licenses are shared by host. That means all Arnold instances running on the same computer share the same license.
For example, you can have multiple copies of Houdini running on your computer, all doing render regions with HtoA, and you’ll be taking just a single Arnold license. Same thing with Softimage and Maya.
Even when licenses are shared, you will still see multiple checkouts in your license usage. For example, here’s my license usage when I have an IPR going in Maya, and render regions in Softimage and Houdini:
The debug log would show something similar:
It might look like there are three licenses checked out, but it’s not. It’s just one license, checked out three times (aka shared three times).
And if you have reporting enabled, you can check the report and see that all three licenses have the same share handle (which means that the license is being shared):