OpenIddict features three independent stacks:
A server stack, that allows creating an OAuth 2.0/OpenID Connect server instance that can be used with internal or external clients. The server stack can be used in both ASP.NET 4.6.1+ and ASP.NET Core 2.1+ applications. To get started, read Creating your own server instance.
A validation stack, that allows implementing token authentication support for your ASP.NET 4.6.1+ and ASP.NET Core 2.1+ APIs. To get started, read Implementing token validation in your APIs.
A client stack, that allows integrating with internal or external OAuth 2.0/OpenID Connect servers. The client stack can be used in ASP.NET 4.6.1+ or ASP.NET Core 2.1+ applications and can also be used in non-web applications (e.g Windows/Linux console applications or Windows desktop applications). To get started, read Integrating with a remote server instance.
The three stacks can be used together or independently, depending on your specific scenario.