diff --git a/specification/trace/sdk.md b/specification/trace/sdk.md
index 4a361033eaf..ba8b00a6abd 100644
--- a/specification/trace/sdk.md
+++ b/specification/trace/sdk.md
@@ -119,6 +119,10 @@ These are the default samplers implemented in the OpenTelemetry SDK:
   that are root spans (no parent) and Spans with remote parent. However there
   should be configuration to change this to "root spans only", or "all spans".
   * Description MUST be `ProbabilitySampler{0.000100}`.
+  * If the probability is applied (as opposed to using the parent's
+  `SampledFlag` value), the set of span Attributes returned in the result MUST
+  include an attribute with the key `sampling.probability` and a value of the
+  probability being used.
 
 #### Probability Sampler algorithm