diff --git a/draft-ietf-ppm-dap.md b/draft-ietf-ppm-dap.md
index 57a76884..5607c05d 100644
--- a/draft-ietf-ppm-dap.md
+++ b/draft-ietf-ppm-dap.md
@@ -566,11 +566,12 @@ is to compute `agg_result = F(agg_param, meas_1, ..., meas_N)` for some
 function `F` while revealing nothing else about the measurements. We call `F`
 the "aggregation function" and `agg_result` the "aggregate result".
 
-DAP is extensible in that it allows for the addition of new cryptographic
-schemes that compute different aggregation functions. In particular, the
-aggregation function is determined by the Verifiable Distributed Aggregation
-Function, or VDAF {{!VDAF=I-D.draft-irtf-cfrg-vdaf-12}}, used to securely
-compute it.
+cryptographic schemes that compute different aggregation functions. In
+particular, the aggregation function is determined by the Verifiable Distributed
+Aggregation Function, or VDAF {{!VDAF=I-D.draft-irtf-cfrg-vdaf-12}}, used to
+securely compute it. This protocol only supports VDAFs which require a single
+collection to provide useful results, and whose aggregation results are
+independent of the order in which measurements are aggregated.
 
 VDAFs rely on secret sharing to protect the privacy of the measurements. Rather
 than sending its measurement in the clear, each Client shards its measurement