Describe caller-specific contribution limits in explainer

Issue patcg-individual-drafts#81

README.md

@@ -22,8 +22,8 @@ Author: Alex Turner ([email protected])
     - [Duplicate debug report](#duplicate-debug-report)
   - [Reducing volume by batching](#reducing-volume-by-batching)
     - [Batching scope](#batching-scope)
-    - [Contributions limit](#contributions-limit)
     - [Padding](#padding)
+    - [Limiting the number of contributions per report](#limiting-the-number-of-contributions-per-report)
   - [Aggregation coordinator choice](#aggregation-coordinator-choice)
 - [Privacy and security](#privacy-and-security)
   - [Metadata readable by the reporting origin](#metadata-readable-by-the-reporting-origin)
@@ -380,21 +380,32 @@ One consideration in the short term is that these calls may have different
 associated [debug modes or keys](#temporary-debugging-mechanism). In this case,
 only calls sharing those details should be batched together.
 
-#### Contributions limit
+#### Limiting the number of contributions per report
 
 We will also need a limit on the number of contributions within a single report.
 In the case that too many contributions are specified with a ‘batching scope’,
-we should truncate them to the limit.
-
-However, to reduce the impact of this limit, we will pre-aggregate (i.e. merge)
-any contributions that have the same bucket and [filtering
+we should truncate them to the limit. To reduce the impact of this limit, we
+will pre-aggregate (i.e. merge) any contributions that have the same bucket and
+[filtering
 ID](https://github.com/patcg-individual-drafts/private-aggregation-api/blob/main/flexible_filtering.md#proposal-filtering-id-in-the-encrypted-payload)
 before truncation.
 
-If necessary, we could instead split the contributions back into multiple
-reports, each respecting the limit.
+Some callers may benefit from different limits. Shared Storage callers can be
+resilient to a low limit because they can simply hold onto any excess
+contributions until their next invocation. On the other hand, Protected Audience
+callers are attached to an auction, so any truncated contributions are
+unrecoverable.
+
+Conveniently, it's not necessary for the limit to be a constant, so long as it
+is determined independently of cross-site data. It follows that the limit may be
+chosen based on the the calling API's identity without any affect on privacy.
+More complex designs that enable finer-grained configuration are also possible,
+but require further analysis (see [issue #81]).
+
+[issue #81]: https://github.com/patcg-individual-drafts/private-aggregation-api/issues/81
 
-Strawman limit: 20 contributions per report.
+We suggest a default of 20 contributions per report with a carveout of 100
+contributions per report for Protected Audience callers.
 
 #### Padding