diff --git a/specification/gedcom-3-structures-3-meaning.md b/specification/gedcom-3-structures-3-meaning.md
index a34edb66..eabd3c9e 100644
--- a/specification/gedcom-3-structures-3-meaning.md
+++ b/specification/gedcom-3-structures-3-meaning.md
@@ -437,6 +437,13 @@ with substructures providing additional details about the source (not the export
 The principal date of the subject of the superstructure.
 The payload is a `DateValue`.
 
+When the superstructure is an event, the principal date indicates when the event took place.
+
+When the superstructure is an attribute, the principal date indicates when the attribute was observed, asserted, or applied.
+A date period might put bounds on the attributes applicability, but other date forms assume that the attribute may have also applied on other dates too.
+
+When the superstructure is a `g7:SOUR-DATA`, the principal date indicates when the data was entered into the source; or, for a source like a website that changes over time, a date on which the source contained the data.
+
 See `DATE_VALUE` for more details.
 
 #### `DATE` (Date) `g7:DATE-exact`