Index: 04_attrib_imp.html
===================================================================
--- 04_attrib_imp.html	(revision 7801)
+++ 04_attrib_imp.html	(revision 7802)
@@ -240,3 +240,22 @@
 -slot=3. Minus prefixed conrete attributes are ignored during compilation,
 the resulting component does not have a slot attribute and there's no collision.
 
+
+<h3> 4.4.4. safe keys for user attributes </h3>
+<p>{imp4:28}
+Users would store their own attributes in groups, especially in symbols
+and wirenets. They would choose arbitrary attribute keys that do not
+clash with current attribute keys used by the code of a given
+implementation (e.g. sch-rnd) at the given time. However, future
+versions of the same implementation or a different implementation could
+use that same attribute key for other purposes, causing a clash, making
+the user's sheet non-portable.
+
+<p>{imp4:29}
+To avoid this problem, the convention is that attribute key prefix <b>usr/</b>
+is reserved for user attributes: no code shall depend on such attributes.
+Such attributes shall be used exclusively by the user, e.g. on the GUI
+or exported to netlists and BOMs.
+
+
+