diff --git a/specs/agents/tracing-spans.md b/specs/agents/tracing-spans.md
index f856ff6a..d945c7d9 100644
--- a/specs/agents/tracing-spans.md
+++ b/specs/agents/tracing-spans.md
@@ -33,6 +33,17 @@ Each span will have a `name`, which is a descriptive, low-cardinality string.
 
 If a span is created without a valid `name`, the string `"unnamed"` SHOULD be used.
 
+### Span `sync`
+
+Span execution within a transaction or span can be synchronous (the caller waits for completion), or asynchronous (the caller does not wait
+for completion).
+
+In UI:
+
+- when `sync` field is not present or `null`, we assume it's the platform default and no badge is shown.
+- when `sync` field is set to `true`, a `blocking` badge is shown in traces where the platform default is `async`: `nodejs`, `rum` and `javascript`
+- when `sync` field is set to `false`, an `async` badge is shown in traces where the platform default is `blocking`: other agents
+
 ### Span outcome
 
 The `outcome` property denotes whether the span represents a success or failure, it is used to compute error rates