Describe per-context limits in explainer

README.md

@@ -389,16 +389,33 @@ will 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.
 
-This limit may vary by caller. In particular, Protected Audience reports may
-benefit from a higher limit more than Shared Storage reports.
-
-More complex designs that enable callers to configure custom limits are also
-possible, but require further analysis (see [issue #81]).
-
-[issue #81]: https://github.com/patcg-individual-drafts/private-aggregation-api/issues/81
-
-Our implementation plan is to set the limit at 20 contributions per report for
-Shared Storage and 100 contributions per report for Protected Audience.
+Selecting the limit's value presents a tradeoff. Although larger reports have
+higher utility, they are also more expensive to process on the aggregation
+service. To accommodate use cases with diverse utility requirements and cost
+tolerances, we define several limit-selection strategies below.
+
+- *One global limit:* The simplest strategy is to define a single limit that the
+  browser applies to all reports. Our implementation currently enforces a limit
+  of 20 contributions per report.
+
+- *Per-API limit:* The browser may select the limit based on the identity of the
+  calling API. In particular, Protected Audience reports may benefit from a
+  higher limit more than Shared Storage reports. Our implementation plan is to
+  set the limit at 20 contributions per report for Shared Storage and 100
+  contributions per report for Protected Audience.
+
+- *Per-context limit:* Callers may request a different limit on each isolated
+  context they create. However, callers that possess cross-site information
+  cannot use this mechanism. Consequently, Protected Audience buyers cannot set
+  per-context limits. The browser must clamp excessively large values to some
+  maximum value. Our implementation plan is to clamp the requested limit to a
+  maximum of 1000 contributions per report.
+
+- *Per-site limit:* A more complex design that enables sites to configure a
+  global limit may also be possible, but requires further analysis. (See [issue
+  #81].)
+
+  [issue #81]: https://github.com/patcg-individual-drafts/private-aggregation-api/issues/81
 
 #### Padding