added info about UDC, #728

_posts/2024-03-20-iso-old-layouts.adoc

@@ -136,6 +136,18 @@ More information about boilerplate predefined text see
 at https://www.metanorma.org/develop/topics/metadata-and-boilerplate/#boilerplate
 
 
+
+== Universal Decimal Classification (UDC) codes support
+
+ISO used UDC instead of ICS in publications prior to 1994 years. You can specify
+the UDC code(s) in the document attribute :classification:. Values are prefixed 
+with `UDC:`, e.g.:
+
+```
+:classification: UDC:536.5.081:531.71
+```
+
+
 == Two-columns layout
 
 Some old ISO standards published in the two-columns layout. You cas switch the default