This is a general discussion of how to customize the fields that are displayed in the registration form based on the page or the post that it is being displayed on.
There are some more advanced ways to approach this, but for most users, this will be the easiest to implement.
Note: this is a general discussion of how to implement. It is not a copy/paste codes snippet (although the code examples can be copy/pasted and used as-is as a general overview of trying out the approach discussed). You will need a basic knowledge of PHP to implement this based on your own custom fields.