Statistics README

Author: davxy

statistics/README.md

@@ -1,8 +1,17 @@
-# Statistics STF Test Vectors
+# Validators Statistics STF Test Vectors
 
 Both JSON and SCALE formats conform to the JAM ASN.1 [schema](../jam-types-asn/jam-types.asn)
 and this subsystem STF specific [schema](./statistics.asn).
 
+## Validators Statistics
+
+These vectors exclusively contribute to updating validator-related statistics
+($π_V$ and $π_L$).
+
+In contrast, service and core statistics ($π_S$ and $π_C$) are updated by
+vectors that more directly influence the state changes relevant to those
+metrics (i.e. see `preimages`, `reports` and `accumulate`).
+
 ## ⚠️Extrinsic Semantic Validity
 
 These vectors are intended just to advance the statistics of validators.