Log in with Congressus (through OAuth2 and OpenID Connect) is meant for letting your association's members log in. You can use it to make sure only members can log in to your application. You can think of it as "working on behalf of a member".

The Congressus API is meant to open up functionality you find in Congressus Manager to your application, such as creating a sale.

You can use both in one application: first let the member log in through Log in with Congressus, then fetch the member's relevant details through the Congressus API.