Add substituting tag, remove some old code

Docs/Tags_format.md

@@ -35,3 +35,32 @@ Regex equals
 A tag can also be tested against a regex with `key~regex`. Note that this regex __must match__ the entire value. If the value is allowed to appear anywhere as substring, use `key~.*regex.*`
 
 Equivalently, `key!~regex` can be used if you _don't_ want to match the regex in order to appear.
+
+
+Using other tags as variables
+-----------------------------
+
+**This is an advanced feature - use with caution**
+
+Some tags are automatically set or calculated - see [CalculatedTags](CalculatedTags.md) for an entire overview.
+If one wants to apply such a value as tag, use a substituting-tag such, for example`survey:date:={_date:now}`. Note that the separator between key and value here is `:=`.
+The text between `{` and `}` is interpreted as a key, and the respective value is substituted into the string.
+
+One can also append, e.g. `key:={some_key} fixed text {some_other_key}`.
+
+An assigning tag _cannot_ be used to query OpenStreetMap/Overpass.
+
+If using a key or variable which might not be defined, add a condition in the mapping to hide the option.
+This is because, if `some_other_key` is not defined, one might actually upload the literal text `key={some_other_key}` to OSM - which we do not want.
+
+To mitigate this, use:
+
+```
+"mappings": [
+{
+    "if":"key:={some_other_key}"
+    "then": "...",
+    "hideInAnswer": "some_other_key="
+}
+]
+```