Merge pull request #436 from w3c/timg-agg-svc-detail

Provide more detail on the nature of aggregation services

Author: martinthomson

api.bs

@@ -315,6 +315,9 @@ path:images/overview.svg
 <figcaption>Overview of Attribution Operation</figcaption>
 </figure>
 
+
+## Recording Impressions ## {#overview-impression}
+
 When an [=impression=] occurs,
 the <a method for=Attribution>saveImpression()</a> method can be used
 to request that the browser save information.
@@ -323,6 +326,9 @@ and some additional information about the impression.
 For instance, advertisers might use additional information
 to record whether the impression was an ad view or an ad click.
 
+
+## Conversions or Querying Impressions ## {#overview-conversion}
+
 At [=conversion=] time, a [=conversion report=] is created.
 A <dfn>conversion report</dfn> is an encrypted histogram contribution
 that includes information from any [=impressions=] that the browser previously stored.
@@ -354,6 +360,9 @@ This minimally involves encryption of the histogram.
 <p class=note>A site that invokes this API will always receive a valid conversion report.
 As a result, sites learn nothing about what happened on other sites from this interaction.
 
+
+## Aggregation ## {#overview-aggregation}
+
 The site can collect the encrypted histograms it receives from calls to this API
 and submit them to the aggregation service.
 
@@ -369,7 +378,21 @@ Upon receiving a set of encrypted histograms from a site, the aggregation servic
 
 3.  returns the aggregate to the site.
 
-We refer to the final output as an <dfn>attribution result</dfn>.
+We refer to the final output of the aggregation service as an <dfn>attribution result</dfn>.
+
+Browser implementations need to have a set of aggregation services that are trusted
+to reliably perform the above functions.
+From the set of services that are offered,
+the [=conversion site=] needs to reach an agreement
+with their choice of service to perform aggregation.
+
+Browsers might establish programs for registering aggregation services,
+similar in nature to the [root store program](https://www.ccadb.org/)
+used to authorize the certification authorities
+that browsers use in the validation
+of Web <abbr title="Public Key Infrastructure">PKI</abbr> certificates.
+The establishment of such a program
+is out of scope for this document.
 
 
 # API Usage # {#api}
@@ -528,7 +551,7 @@ The URL is passed as the {{AttributionConversionOptions/aggregationService}} par
 to <a method for=Attribution>measureConversion()</a> to select the identified aggregation service.
 
 The <dfn enum>AttributionAggregationProtocol</dfn> describes the submission protocol
-used by different [=aggregation services=]. This document defines two protocols:
+used by different [=aggregation services=]. This document defines one protocol:
 
 <dl dfn-for=AttributionAggregationProtocol dfn-type=enum-value>
   <dt><dfn>dap-18-histogram</dfn></dt>