diff --git a/specification/context/api-propagators.md b/specification/context/api-propagators.md
index d366ff93df9..e00124b49f2 100644
--- a/specification/context/api-propagators.md
+++ b/specification/context/api-propagators.md
@@ -18,6 +18,7 @@ Table of Contents
       - [Set](#set)
   - [Extract](#extract-1)
     - [Getter argument](#getter-argument)
+      - [Keys](#keys)
       - [Get](#get)
 - [Composite Propagator](#composite-propagator)
 - [Global Propagators](#global-propagators)
@@ -116,7 +117,7 @@ avoid runtime allocations.
 
 ### Fields
 
-The propagation fields defined. If your carrier is reused, you should delete the fields here
+The predefined propagation fields. If your carrier is reused, you should delete the fields here
 before calling [inject](#inject).
 
 Fields are defined as string keys identifying format-specific components in a carrier.
@@ -132,6 +133,10 @@ The use cases of this are:
 
 Returns list of fields that will be used by the `TextMapPropagator`.
 
+Observe that some `Propagator`s may define, besides the returned values, additional fields with
+variable names. To get a full list of fields for a specific carrier object, use the
+[Keys](#keys) operation.
+
 ### Inject
 
 Injects the value into a carrier. The required arguments are the same as defined by
@@ -180,7 +185,23 @@ Getter is an argument in `Extract` that get value from given field
 
 `Getter` allows a `TextMapPropagator` to read propagated fields from a carrier.
 
-One of the ways to implement it is `Getter` class with `Get` method as described below.
+One of the ways to implement it is `Getter` class with `Get` and `Keys` methods
+as described below. Languages may decide on alternative implementations and
+expose corresponding methods as delegates or other ways.
+
+##### Keys
+
+The `Keys` function MUST return the list of all the keys in the carrier.
+
+Required arguments:
+
+- The carrier of the propagation fields, such as an HTTP request.
+
+The `Keys` function can be called by `Propagator`s which are using variable key names in order to
+iterate over all the keys in the specified carrier.
+
+For example, it can be used to detect all keys following the `uberctx-{user-defined-key}` pattern, as defined by the
+[Jaeger Propagation Format](https://www.jaegertracing.io/docs/1.18/client-libraries/#baggage).
 
 ##### Get