URL Terms in LTI "Tool" Test application

Hi all.

I'm working on an LTI1.3 integration for a small, open-source LMS. In setting up a test tool on https://lti-ri.imsglobal.org/lti/tools/ (thanks for this resource, 1EdTech), I'm a bit confused about the terms for the fields.

The new tool form has a field for
- OAuth2 URL:
- Platform OIDC Auth URL

As I understand the LTI 1.3 workflow, after the pre-flight launch from the platform to the tool, there's only *one* redirect from the tool back to the platform. After that redirect from tool back to the platform, the platform constructs and provides an ID Token and redirects back to the tool so it can show the intended resource.

If I have that right, which of those two URLs shown above -- "OAuth2 URL" or "Platform OIDC Auth URL" is means to be used for this callback URL on the platform?

I would assume "Platform OIDC Auth URL", but then why do I need to define a "OAuth2 URL" for my platform? Can that be safely left empty?

Thanks for any thoughts!! (Apologies if this has been answered before. I'm struggling to find a way to search these forums. There's no immediate search option, and the site's global search is currently showing an error.)