docs: add concept of variables priority

docs/user/concepts.md

@@ -0,0 +1,72 @@
+
+## variables priority
+
+There are several different types of `variables`, and the priory can be confusing. The best way to avoid confusion is to use different variable names. However, if you must use the same variable names, you should understand the priority strategy.
+
+### testcase
+
+```yaml
+config:
+    name: xxx
+    variables:              # config variables
+        varA: "configA"
+        varB: "configB"
+        varC: "configC"
+    parameters:             # parameter variables
+        varA: ["paramA1"]
+        varB: ["paramB1"]
+
+teststeps:
+-
+    name: step 1
+    variables:              # step variables
+        varA: "step1A"
+    request:
+        url: /$varA/$varB/$varC # varA="step1A", varB="paramB1", varC="configC"
+        method: GET
+    extract:                # extracted variables
+        varA: body.data.A   # suppose varA="extractVarA"
+        varB: body.data.B   # suppose varB="extractVarB"
+-
+    name: step 2
+    varialbes:
+        varA: "step2A"
+    request:
+        url: /$varA/$varB/$varC # varA="step2A", varB="extractVarB", varC="configC"
+        method: GET
+```
+
+In a testcase, variables priority are in the following order:
+
+- step variables > extracted variables, e.g. step 2, varA="step2A"
+- parameter variables > config variables, e.g. step 1, varB="paramB1"
+- extracted variables > parameter variables > config variables, e.g. step 2, varB="extractVarB"
+- config variables are in the lowest priority, e.g. step 1/2, varC="configC"
+
+### testsuite
+
+```yaml
+config:
+    name: xxx
+    variables:                  # testsuite config variables
+        varA: "configA"
+        varB: "configB"
+        varC: "configC"
+
+testcases:
+-
+    name: case 1
+    variables:                  # testcase variables
+        varA: "case1A"
+    testcase: /path/to/testcase1
+    export: ["varA", "varB"]    # export variables
+-
+    name: case 2
+    varialbes:                  # testcase variables
+        varA: "case2A"
+    testcase: /path/to/testcase2
+```
+
+In a testsuite, variables priority are in the following order:
+
+- testcase variables > export variables > testsuite config variables > referenced testcase config variables

mkdocs.yml

@@ -54,6 +54,7 @@ nav:
     - Introduction: index.md
     - Installation: installation.md
     - User Guide:
+        - Concepts: user/concepts.md
         - Scaffold: user/scaffold.md
         - Record & Generate testcase: user/gen_tests.md
         - Write Testcase: user/write_testcase.md