- * UST. Fields need to be only added at the end, never reordered, never
- * removed.
- *
- * The field @struct_size should be used to determine the size of the
- * structure. It should be queried before using additional fields added
- * at the end of the structure.
+ * UST. Additional selectors may be added in the future, mapping to new
+ * union fields, which means the overall size of this structure may
+ * increase. This means this structure should never be nested within a
+ * public structure interface, nor embedded in an array.