In this example, the matrix entries for node-version are each configured to use different values for the site and datacenter environment variables. The Echo site details step then uses {% raw %}env: ${{ matrix.env }}{% endraw %} to refer to the custom variables:

{% raw %}

name: Node.js CI
on: [push]
jobs:
  build:
    runs-on: ubuntu-latest
    strategy:
      matrix:
       include:
         - node-version: 10.x
           site: "prod"
           datacenter: "site-a"
         - node-version: 12.x
           site: "dev"
           datacenter: "site-b"
    steps:
      - name: Echo site details
        env:
          SITE: ${{ matrix.site }}
          DATACENTER: ${{ matrix.datacenter }}
        run: echo $SITE $DATACENTER

{% endraw %}