diff --git a/i-d/pdns-qof.xml b/i-d/pdns-qof.xml
index 613c644..ef371e4 100644
--- a/i-d/pdns-qof.xml
+++ b/i-d/pdns-qof.xml
@@ -234,7 +234,8 @@ ws              = *(
 		<section title="Additional Fields">
 			<t>Implementations MAY support the following fields:</t>
 			<section title="sensor_id">
-			<t>This field returns the sensor information where the record was seen. It is represented as a <xref target="RFC4627">JSON</xref> string.</t>
+                    <t>This field returns the sensor information where the record was seen. It is represented as a <xref target="RFC4627">JSON</xref> string.</t>
+                    <t>If the data originate from sensors or probes which are part of a publicly-known gathering or measurement system (e.g. RIPE Atlas), a <xref target="RFC4627">JSON</xref> string SHOULD be prefixed.</t>
 		    </section>
             <section title="zone_time_first">
             <t>This field returns the first time that the unique tuple (rrname, rrtype, rdata) record has been seen via master file import. The date is expressed in seconds (decimal) since 1st of January 1970 (Unix timestamp). The time zone MUST be UTC. This field is represented as a <xref target="RFC4627">JSON</xref> number.</t>