Explain when to use default variables in a context and an environment (#16650)

* Added "Determining which type of environment variables to use"

* Moved into reusable, added to various locations

* Added some small edits

* Some rewording

* Revised example

* Removed reusable reference from one location

* Update context-and-expression-syntax-for-github-actions.md

* Fixed merge conflict

* Update environment-variables.md

* Apply suggestions from code review

Co-authored-by: Sarah Edwards <skedwards88@github.com>

* Update using-context-or-environment-variables.md

* Clarified locations of context variables

* Clarified the distinction between contexts and default environment variables

* Moved section up

* Clarified that _most_ contexts can be used anywhere

Co-authored-by: Sarah Edwards <skedwards88@github.com>

Author: Martin Lopes

content/actions/reference/context-and-expression-syntax-for-github-actions.md

@@ -75,6 +75,10 @@ In order to use property dereference syntax, the property name must:
 - start with `a-Z` or `_`.
 - be followed by `a-Z` `0-9` `-` or `_`.
 
+#### Determining when to use contexts
+
+{% data reusables.github-actions.using-context-or-environment-variables %}
+
 #### `github` context
 
 The `github` context contains information about the workflow run and the event that triggered the run. You can read most of the `github` context data in environment variables. For more information about environment variables, see "[Using environment variables](/actions/automating-your-workflow-with-github-actions/using-environment-variables)."

content/actions/reference/environment-variables.md

@@ -57,6 +57,10 @@ We strongly recommend that actions use environment variables to access the files
 | `GITHUB_API_URL` | Returns the API URL. For example: `{% data variables.product.api_url_code %}`.
 | `GITHUB_GRAPHQL_URL` | Returns the GraphQL API URL. For example: `{% data variables.product.graphql_url_code %}`.
 
+#### Determining when to use default environment variables or contexts
+
+{% data reusables.github-actions.using-context-or-environment-variables %}
+
 ### Naming conventions for environment variables
 
 {% note %}

data/reusables/github-actions/using-context-or-environment-variables.md

@@ -0,0 +1,21 @@
+{% data variables.product.prodname_actions %} includes a collection of variables called _contexts_ and a similar collection of variables called _default environment variables_. These variables are intended for use at different points in the workflow:
+
+- **Default environment variables:** These variables exist only on the runner that is executing your job. For more information, see "[Default environment variables](/actions/reference/environment-variables#default-environment-variables)."
+- **Contexts:** You can use most contexts at any point in your workflow, including when _default environment variables_ would be unavailable. For example, you can use contexts with expressions to perform initial processing before the job is routed to a runner for execution; this allows you to use a context with the conditional `if` keyword to determine whether a step should run. Once the job is running, you can also retrieve context variables from the runner that is executing the job, such as `runner.os`. For more information, see "[Contexts](/actions/reference/context-and-expression-syntax-for-github-actions#contexts)."
+
+The following example demonstrates how these different types of environment variables can be used together in a job:
+
+{% raw %}
+```yaml
+name: CI
+on: push
+jobs:
+  prod-check:
+    if: github.ref == 'refs/heads/main'
+    runs-on: ubuntu-latest
+    steps:
+      - run: echo "Deploying to production server on branch $GITHUB_REF"
+```
+{% endraw %}
+
+In this example, the `if` statement checks the [`github.ref`](/actions/reference/context-and-expression-syntax-for-github-actions#github-context) context to determine the current branch name; if the name is `refs/heads/main`, then the subsequent steps are executed. The `if` check is processed by {% data variables.product.prodname_actions %}, and the job is only sent to the runner if the result is `true`. Once the job is sent to the runner, the step is executed and refers to the [`$GITHUB_REF`](/actions/reference/environment-variables#default-environment-variables) environment variable from the runner.