add Backend Calculation Engine

_data/navigation.yml

@@ -156,6 +156,8 @@ dev-ref:
     children:
     - title: User Permission
       url: /dev-ref/User_Permission
+    - title: Backend Calculation Engine
+      url: /dev-ref/Backend_Calculation_Engine
     - title: More Use Cases
       url: https://keikai.io/blog/category/usecase/      
   - title: FAQ

dev-ref/Backend_Calculation_Engine.md

@@ -0,0 +1,32 @@
+---
+title: 'Backend Calculation Engine'
+---
+
+# Overview
+Keikai provides the capability to use spreadsheets in a non-UI, headless fashion, like an automated backend calculation engine.
+
+The key component is the [Book Model](/dev-ref/book_model/Manipulating_Book_Model), which stores all cell data and formulas. You can directly access and manipulate this model programmatically, without needing to render a visual spreadsheet in a browser.
+
+This enables loading data into a Book Model from any source (e.g. databases), executing calculations with formulas, and reading back results, all without any UI.
+
+    Data   ----> Book Model  ----> Result
+
+With this kind of usage, keikai can be a backend calculation engine or perform a automating operation.
+
+## Benefits
+Using Keikai as a backend calculation engine provides these benefits:
+
+* No UI required - Automate spreadsheet workflows without rendering a visual interface.
+* Update calculations via formulas - Formula logic can be modified without code changes, lowering maintenance effort.
+* Leverage spreadsheet calculation power - Harness the calculation capabilities of spreadsheets for backend data processing.
+
+# Steps
+The general steps to use Keikai in headless way are:
+1. [Import an XLSX file](/dev-ref/import) to instantiate a Book model. 
+The file may contain predefined formulas.
+2. [Load input data](/dev-ref/book_model/Manipulating_Book_Model) into the model with `Range` API.
+Recommend to use named range instead of cell address for readability and stability.
+3. Retrieve calculation output by reading cell values using the `Range` API.
+
+
+By eliminating UI concerns, Keikai can serve as a powerful calculation engine for server-side automation processes. The XLSX format provides a portable way to apply spreadsheet logic across many use cases.