According to the information provided in the Spine forums, Spine licenses, including Spine Professional and Spine Essential, are non-transferrable as per the license agreement. Each person using Spine needs their own license, and the license is for a specific, named user and cannot be shared. However, it's mentioned that the Spine team generally doesn't mind changing the ownership (or email address) of a license from time to time, but this is handled on a case-by-case basis. You would need to contact Esoteric Software directly to discuss the possibility of transferring a license temporarily, but based on the provided contexts, it seems that the intended use case of transferring a license temporarily to someone else does not align with the standard license terms.
