diff --git a/databox/README.md b/databox/README.md
index 30ad854f5..723b56afb 100644
--- a/databox/README.md
+++ b/databox/README.md
@@ -36,6 +36,151 @@ Elasticsearch query: "collection:Appl"
 
 ## Metadata
 
+__All__ metadata (except binary blobs) are extracted from file after upload.
+They are saved _as is_ in json in db `file.metadata`.
+
+Each of 21324 known metadata is identified by a unique `TagGroup:TagName`, e.g.
+- `ExifIFD:CreateDate`
+- `GPS:GPSLatitude`
+- `IPTC:Keywords`
+- ...
+
+
+## Initial attribute value(s)
+### _Covers Phraseanet "read metadata"_
+
+The initial value(s) of an attribute is defined by its `attribute-definition: Initial Values All` setting.
+
+The setting is (for now) expressed as `json`, allowing to specify 2 "types" of source value:
+
+- simple unique `metadata` source:
+```json
+{
+    "type": "metadata",
+    "value": "IPTC:Keywords"
+}
+```
+
+- Computation of text, metadata, tests, math, ... using the twig `templating` language.
+
+One can access to file properties, including metadata tags _via_ the `file` object, or also acces to `asset` properties:
+```json
+{
+    "type": "template",
+    "value": "{{ file.filename }} (size={{ file.size }})"
+}
+```
+```json
+{
+    "type": "template",
+    "value": "Copyright: {{ file.metadata('IPTC:Credit').value }} ; Phrasea OwnerId: {{ asset.OwnerId }}"
+}
+```
+Fetch the first available "CreationDate" from multiple possible tags:
+```json
+{
+    "type": "template",
+    "value": "{{ file.metadata('ExifIFD:CreateDate').value ?? file.metadata('IPTC:DateCreated').value ?? file.metadata('IPTC:DigitalCreationDate').value }}"
+}
+```
+To populate a `multi-values` attribute using twig template, one must generate **one item per line**.
+
+Here is how to populate uppercased keywords:
+```json
+{
+    "type": "template",
+    "value": "{% for kw in file.metadata('IPTC:Keywords').values %}{{ kw|upper }}\n{% endfor %}"
+}
+```
+
+### Short list of commonly used tags:
+```text
+Composite:GPSPosition
+ExifIFD:CreateDate
+ExifIFD:DateTimeOriginal
+ExifIFD:ImageUniqueID
+IPTC:By-line
+IPTC:By-lineTitle
+IPTC:Caption-Abstract
+IPTC:CopyrightNotice
+IPTC:City
+IPTC:Country-PrimaryLocationName
+IPTC:CopyrightNotice
+IPTC:Credit
+IPTC:ImageOrientation
+IPTC:Keywords
+JPEG:Comment
+PDF:Author
+PDF:Keywords
+PDF:PageCount
+PDF:Subject
+PDF:Title
+XMP-dc:Date
+XMP-dc:Description
+XMP-dc:Language
+XMP-dc:Publisher
+XMP-dc:Rights
+XMP-dc:Source
+XMP-dc:Subject
+XMP-dc:Title
+XMP-exif:FlashFired
+XMP-iptcExt:PersonInImage
+XMP-xmp:Author
+XMP-xmp:Keywords
+XMP-xmp:Rating
+XMP-xmp:Title
+```
+for complete list, refer to: `databox/api/var/cache/phpexiftool/Helper.php`
+
+:warning: With `initial values` setting, attributes values are created only once, - when a file is added -.
+Changes from dynamic / editable sources (e.g. refering to `asset.title`) will **not** update the related
+attributes.
+
+## Fallback attribute value(s) !! wip do not use !!
+A fallback value defines the "virtual" value of an attribute if this attribute is **not set** for the asset.
+This "virtual" computed value is:
+- searchable - if the attribute definition allows it -
+- readable (returned by api)
+- displayed in databox applications
+- not editable
+
+The fallback value for an attribute is defined **by locale (lng)**, using the same syntax as `initial values`.
+
+The value is computed on asset indexation (thus after editing) and on display.
+
+e.g. fallback for unset "Credit", in EN and FR
+```json
+{
+    "type": "template",
+    "value": "(c) CoolMedia Agency (unknown author)"
+}
+```
+```json
+{
+    "type": "template",
+    "value": "(c) Agence CoolMedia (auteur inconnu)"
+}
+```
+
+Because the fallback "formula" can refer to other (real) attributes values, it is possible to generate attributes
+that depends on other attributes.
+
+e.g.: fill the "warning" attribute if the Title or Credit is not set
+```json
+{
+    "type": "template",
+    "value": "{% if attr('Title') is empty %}Missing Title\n{% endif %}{% if attr('Credit') is empty %}Missing Credit\n{% endif %}"
+}
+```
+
+
+
+
+
+
+
+---
+## old doc to be cleaned
 - Title
 - Length : [00:31:01] [X] Overrive value