diff --git a/specification/glossary.md b/specification/glossary.md
index 443971a6a..e1fb249de 100644
--- a/specification/glossary.md
+++ b/specification/glossary.md
@@ -86,7 +86,7 @@ An open-source specification that defines requirements for billing data.
 
 <a name="glossary:FOCUS-dataset"><b>FOCUS Dataset</b></a>
 
-A structured collection of cost and usage data that meets or exceeds the Basic compliance criteria of FOCUS.
+A structured collection of cost and usage data that meets or exceeds the Basic compliance criteria of FOCUS. In addition to FOCUS columns, the dataset should include custom provider columns (prefixed with `x_`) when these columns provide additional information not captured by the existing FOCUS columns. If introducing a custom column could result in splitting original charge records into multiple entries, the Invoice Issuer is responsible for ensuring that the FOCUS dataset fully conforms to all aggregation-related requirements for metric columns, particularly those concerning costs and quantities.
 
 <a name="glossary:inclusivebound"><b>Inclusive Bound</b></a>