docs: clarify the use of the tracked shipment option (#118)

* docs: clarity the use of the tracked shipment option

* docs: removed trailing space

src/api-reference/06.shipments.md

@@ -189,6 +189,11 @@ for delivery on saturdays.
 
 Only available on <DataType type="platform" id="1" /> and <DataType type="package-type" id="2" /> or <DataType type="package-type" id="6" />.
 This option can be used to make a <DataType type="package-type" id="2" /> or <DataType type="package-type" id="6" /> tracked.
+Option tracked needs to be used with the international mailbox packages and the international small packages.
+There is one exception: a small package shipment to Belgium.
+If you have a PostNL contract that allows you to use international small packages under agreed conditions with PostNL, you can use the tracked option.
+If you don't have such a contract, the small package shipment to Belgium has the same options as a <DataType type="package-type" id="1" />
+shipment to Belgium and the tracked option is not available.
 
 ### 6.A.4 Examples