serverless/docs/guides/variables/cli-options.md

Read this on the main serverless docs site

Reference CLI Options

To reference CLI options that you passed, use the ${opt:<option>} syntax in your serverless.yml configuration file. It is valid to use the empty string in place of <option>. This looks like "${opt:}" and the result of declaring this in your serverless.yml is to embed the complete options object (i.e. all the command line options from your serverless command).

service: new-service
provider: aws
functions:
  hello:
    name: ${opt:stage}-hello
    handler: handler.hello
  world:
    name: ${opt:stage}-world
    handler: handler.world

In the above example, you're dynamically adding a prefix to the function names by referencing the stage option that you pass in the CLI when you run serverless deploy --stage dev. So when you deploy, the function name will always include the stage you're deploying to.