If you are using one registration form but want to register multiple types of users that will be given different roles on your site, then you might want to use a custom field which controls what role to give them. For example, you add a dropdown to your registration form and depending on what value they select, they get assigned a different role.
To get started, make sure the registration form is set to assign a default role for new users:
Then add a custom field you have created in Users > Custom Fields to the registration form, in my example I have a dropdown to capture the County they live in:
Find out they meta key of the field:
Then grab the code below and save to a wpum.php file to your /wp-content/mu-plugins/ directory (you might need to create the mu-plugins directory).
You will need to edit the meta field key to use your field, and also change the value logic so that the role is set to a different role if the field value is set as that value. You can then change the role as needed.
You can alter this logic to cater for fields that have multiple different options and different roles.