+/**
+ * This interface stands for one attribute of a given flow.
+ *
+ * We mainly have two implementations :
+ *
+ * + Primitive attributes (no predefined choice, the end user must compute a
+ * String, a Float, an Integer...)
+ *
+ * + RefSpecAttribute which enable two things + a reference to another object of
+ * the application context + the display of some choices among which the end
+ * user can choose.
+ *
+ * @see org.argeo.slc.core.execution.PrimitiveSpecAttribute
+ * @see org.argeo.slc.core.execution.RefSpecAttribute
+ * @see org.argeo.slc.core.execution.PrimitiveUtils : this class offers some
+ * helper, among others to cast the various type of primitive attribute.
+ */