A. IdentityServer3 docs, samples and source code use OIDC & OAuth2 terms interchangeably to refer to same thing in many areas. I think that's make sense because OIDC introduced as complement & extension for OAuth2.
B. IdentityServer3, STS, OP, OIDC server, OAuth2 server, CSP, IDP and others: means same thing (software that provide/issue tokens to clients) as explained in [Terminology] (http://identityserver.github.io/Documentation/docs/overview/terminology.html).
C. Grants and flows mean same thing, grant was the common term in OAuth2 specs and flow is the common term in OIDC specs.
D. This document will not focus on custom flow/grant.
E. [Important] Choosing wrong flow leads to security threat.