Explanation
Timeline Description automatically generated with medium confidence

Step 1: In the Plug-in Registration tool, select Register New Assembly.
You use the Plug-in Registration tool (PRT) to register your plug-in assemblies and steps.
Registering an assembly is the process of uploading the assembly to the Dataverse database.
Step 2: In the Plug-in Registration tool, Select Register New Step,..PreOperation PreOperation occurs before the main system operation and within the database transaction.
If you want to change any values for an entity included in the message, you should do it here.
Step 3: In the Plug-in Registration tool, Select Register New Image, change the Image type to be a PreImage, and..
If your plug-in step is registered in the PreValidation or PreOperation stages of the execution pipeline, you could use the Organization service to retrieve the current value of the property, but this is not a good practice for performance. A better practice is to define a pre-entity image with your plug-in step registration.
Reference:
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/register-plug-in