This means that if you are using Spine Editor 3.8.99, you must use in your game engine. This is a hard requirement. Attempting to load a file exported from Spine 3.8.99 into a Spine 4.x runtime or vice-versa will lead to errors and broken animations. The golden rule is simple: the first two numbers (the major and minor) of the editor and runtime versions must be identical.
The "3.8" designation refers to the first major iteration of the Spine 3.0 generation, released initially in August 2019. The ".99" suffix indicates it is the final minor patch of that major version. According to Esoteric Software's versioning policy, any release that does not end with "-beta" is considered a stable release. For the editor, 3.8.99 is stable, corresponding to runtime version 3.8. This release is particularly significant as it marks the end of active support for the 3.x generation, meaning no major new features were added after this point, though bug fixes continued to be applied. Spine 3.8.99
