add definition of static vs dynamic

Signed-off-by: Thomas Poignant <[email protected]>

website/src/pages/specification/20241027-flagsets.md

@@ -18,6 +18,9 @@ Description: Flag sets are a way to group flags together.
 > _Source: [Openfeature glossary](https://openfeature.dev/specification/glossary/#flag-set)._
 
 
+- **static flag set**: the flag set is defined in the retriever configuration, we know in advance that this flag set exists.
+- **dynamic flag set**: the flag set is defined in the flag configuration file, we don't know in advance which flag sets exist.
+
 ## Context
 
 As of today, GO Feature Flag is evaluating flag differently for client and server evaluations.