[ADD] double entry basic doc

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double-entry.rst

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 =================================
 Double-Entry Inventory Management
 =================================
+
+In a double-entry inventory, there is no stock input, output (disparition) or
+transformation. Instead, there are only stock moves between locations.
+
+* Inventory: 3 products in Zone 1
+* Reception: 2 products in Zone 1
+* Delivery: 1 product to client
+* Return: 1 product from client
+* Scrap: 1 product  broken in zone 1
+* Inventory Zone 1: loss of 1 product
+* Move: 1 product Zone 1 ➔ Zone 2
+
+Operations
+==========
+
+Stock moves represent the transit of goods and materials between inventory
+locations.
+
+* Manufacturing Order
+* Drop-shipping
+* Picking ➔ Packing ➔ Shipping
+* Inter-Warehouse transfert
+* Loss of product
+* Inventory
+* Reception
+
+Analysis
+========
+
+Inventory analysis can use products count or value (number of products *
+products cost).
+
+For each inventory location, multiple data points can be analysed:
+
+* inventory valuation
+* value creation (difference between the value of manufactured products and
+  the cost of raw materials used during manufacturing)
+* value of lost/stolen products
+* value of scrapped products
+* value of products delivered to clients over a period
+* value of products in transit between locations
+
+Procurements & Pull Rules
+=========================
+
+A procurement is a request for a specific quantity of products to a specific
+location. They can be created manually or automatically triggered by:
+
+* sale orders
+* minimum stock rules
+* rules
+
+*Pull rules* describe how to fulfill procurements on specific locations:
+
+* where the product should come from (source location)
+* whether the procurement is :abbr:`MTO (Made To Order)` or :abbr:`MTS (Made
+  To Stock)`
+
+.. h:div:: force-right
+
+   .. todo:: needs schema thing from FP
+
+Routes
+======
+
+At each step or a procurement's fulfillment, multiple rules may be
+available. *Routes* define which rules should be used based on the environment
+(product, sales orders, warehouse, …). To fulfill a procurement, the system
+will search for routes in the following order:
+
+1. sale order line routes
+2. product routes
+3. product category routes
+4. warehouse routes
+
+Push Rules
+==========
+
+Push rules are triggered when a product enters a specific location, and allows
+chaining locations. Push rules can also be configured and filtered using
+routes.
+
+Some example:
+* quality control
+* transit warehouse 1
+
+.. warning:: push rules and pull rules are *not* symmetrical, pull rules are
+             triggered by procurement requests whereas push rules are
+             triggered by stock moves
+
+Procurement Groups
+==================
+