When you create a new automation on your site, you're presented with a variety of actions to perform after your automation is triggered. The actions available to you depend on which apps you have installed on your site. However, you also have the choice to create your own action, independent of any app.
The Velo Action service plugin (formerly SPI) allows you to provide your own actions for automations on your site. You can use the service plugin to write code for a custom action instead of selecting a premade action. The action you create is specific for your site and cannot be used on other sites.