diff --git a/core/test/docs.toml b/core/test/docs.toml
index 77353f580..9cd0e31fc 100644
--- a/core/test/docs.toml
+++ b/core/test/docs.toml
@@ -2,6 +2,10 @@
 # can also be set to a date-time like 1979-05-27T07:32:00
 starttime = 2019-01-01 # required
 endtime = 2021-01-01   # required
+
+# Coordinate Reference System
+# The accepted strings are documented here:
+# https://proj.org/en/9.4/development/reference/functions.html#c.proj_create
 crs = "EPSG:4326"      # required
 
 # input files