added comment.

src/VisualStudio/Core/Def/Implementation/FindReferences/Contexts/AbstractTableDataSourceFindUsagesContext.cs

@@ -274,6 +274,7 @@ protected async Task<(Guid, string projectName, SourceText)> GetGuidAndProjectNa
                 var mappedDocumentSpan = await AbstractDocumentSpanEntry.TryMapAndGetFirstAsync(documentSpan, sourceText, CancellationToken).ConfigureAwait(false);
                 if (mappedDocumentSpan == null)
                 {
+                    // this will be removed from the result
                     return null;
                 }
 

src/VisualStudio/Core/Def/Implementation/FindReferences/Contexts/WithoutReferencesFindUsagesContext.cs

@@ -87,6 +87,7 @@ protected override async Task OnDefinitionFoundWorkerAsync(DefinitionItem defini
                 var mappedDocumentSpan = await AbstractDocumentSpanEntry.TryMapAndGetFirstAsync(documentSpan, sourceText, CancellationToken).ConfigureAwait(false);
                 if (mappedDocumentSpan == null)
                 {
+                    // this will be removed from the result
                     return null;
                 }
 

src/VisualStudio/Core/Def/Implementation/FindReferences/Entries/AbstractDocumentSpanEntry.cs

@@ -107,6 +107,8 @@ public override bool TryCreateColumnContent(string columnName, out FrameworkElem
                     return new MappedSpanResult(documentSpan.Document.FilePath, sourceText.Lines.GetLinePositionSpan(documentSpan.SourceSpan), documentSpan.SourceSpan);
                 }
 
+                // if span mapping service filtered out the span, make sure
+                // to return null so that we remove the span from the result
                 return results.FirstOrNullable(r => !r.IsDefault);
             }