Open Source has core definitions around the freedoms (the "speech") allowed to people when making use of that source code.

Source available makes the source code available for free (the "beer") along with certain specific freedoms, but not all of the freedoms that would be required to be Open Source.

Whether or not you think those missing freedoms are important is a matter of personal opinion, I suppose. I think they are, which is why I try to avoid source available software if there is a reasonable open source alternative.

Even Microsoft Windows is source available if you are an important customer: https://www.microsoft.com/en-us/sharedsource/default.aspx

A bit more public and open to anyone is Unreal Engine's source code: https://dev.epicgames.com/documentation/en-us/unreal-engine/...