diff --git a/src/database.py b/src/database.py
index a210fc9..1efe8b3 100644
--- a/src/database.py
+++ b/src/database.py
@@ -14,6 +14,18 @@ import error
 import timespan
 
 KEY_WHO = 'other_entity'
+"""str: Key for transactions to document who this transaction dealt with.
+Usually this is a person or a shop.
+
+Usage: some_transaction[KEY_WHO] = "Bioladen"
+"""
+
+KEY_SOURCE = 'source'
+"""str: Key for transactions to document where the data comes from (import from
+older table, import from bank website, manual typing, ...)
+
+Usage: some_transaction[KEY_SOURCE] = "manual"
+"""
 
 
 def merge_transactions(book: piecash.Book, transactions: Sequence[piecash.Transaction]):