Why does `_tag` exist in the encoded result of tagged error created with Schema.TaggedError?

export class SomeError extends Schema.TaggedError<SomeError>(
    "SomeError",
)("SomeError", {
  field: Schema.Boolean,
}) {}

const error = new SomeError({ field: true });
console.log(Schema.encodeSync(error)); // prints { readonly _tag: "SomeError"; readonly field: boolean; }