diff --git a/src/app/db/schema/projects/schema.md b/src/app/db/schema/projects/schema.md
index a8e5d89d..9facb0f2 100644
--- a/src/app/db/schema/projects/schema.md
+++ b/src/app/db/schema/projects/schema.md
@@ -337,6 +337,8 @@ This more accurately reflects the business process and provides a natural locati
 
 Evidence requirements define what documentation is expected.
 
+The kind of document expected is expressed with the existing `file_type` enum (see the Evidence Requirement table), so the required and submitted document kinds share one taxonomy. Trade-off: because it's an enum rather than the originally proposed `document_type` reference table, introducing a new *required* document kind is a schema change (add an enum value) rather than a data insert.
+
 Documents actually submitted are rows in the existing `uploaded_files` table, linked back to the requirement, workstream and contractor assignment via new nullable FK columns. This reuses the established upload pipeline instead of introducing a parallel file store.
 
 Separating expectation from submission provides flexibility for future enhancements such as: