Additional comment

src/Microsoft.TestPlatform.ObjectModel/DataCollector/IDataCollectorAttachmentProcessor.cs

@@ -22,6 +22,18 @@ public interface IDataCollectorAttachmentProcessor
 
         /// <summary>
         /// Indicates whether attachment processor is supporting incremental processing of attachments
+        /// 
+        /// `SupportsIncrementalProcessing` should indicate if attachment processor is supporting incremental processing of attachments. It means that `ProcessAttachmentSetsAsync` should be [associative](https://en.wikipedia.org/wiki/Associative_property).
+        ///
+        /// If `SupportsIncrementalProcessing` is `True` Test Platform may try to speed up whole process by reprocessing data collector attachments as soon as possible when any two test executions are done.For example let's assume we have 5 test executions which are generating 5 data collector attachments: `a1`, `a2`, `a3`, `a4` and `a5`. Test platform could perform invocations:
+        /// * `var result1 = await ProcessAttachmentSetsAsync([a1, a2, a3], ...);` when first 3 executions are done
+        /// * `var result2 = await ProcessAttachmentSetsAsync(result1.Concat([a4]), ...);` when 4th execution is done
+        /// * `var finalResult = await ProcessAttachmentSetsAsync(result2.Concat([a5]), ...);` when last test execution is done
+        /// 
+        /// If `SupportsIncrementalProcessing` is `False` then Test Platform will wait for all test executions to finish and call `ProcessAttachmentSetsAsync` only once:
+        /// * `var finalResult = await ProcessAttachmentSetsAsync([a1, a2, a3, a4, a5], ...);`
+        /// 
+        /// By default `SupportsIncrementalProcessing` should be `False`, unless processing can take longer time and it's beneficial to start the process as soon as possible.
         /// </summary>
         bool SupportsIncrementalProcessing { get; }