WebAssembly64 lets Unity Web builds grow the WebAssembly heap up to 16 GB by using the WebAssembly Memory64 extension. Although 64-bit pointers allow a much larger address space, Unity Web builds are currently limited to 16 GB by the WebAssembly JavaScript API specification.

Use WebAssembly64 only if your application requires more than 4 GB of memory. This feature can increase code size and runtime overhead.

If your project uses native or JavaScript plug-ins, make them compatible with WebAssembly64 before you build. For more information, refer to Web native plug-ins for Emscripten and Call JavaScript functions from Unity C# scripts.