Non commercial use
Free
Commercial Use
Appears to be free too, but with some conditions on requirring you to identify your OS images with BIOS data and ensure OS updating can happen. See the Commerical Terms of Use from Microsoft. Here's the clause that states its free (updated 2016-11):
License Grant
Subject to the requirements in Section 3 and restrictions in Section 4, Microsoft hereby grants to you a royalty-free, worldwide, non-exclusive, personal, non-transferable, non-assignable, limited license to install a Runtime Image into an Embedded System and distribute your Embedded System to End Users.
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