Let me exercise my understanding and see how wrong I am…
Two OF THE SAME IBL (P1) bred together equals P2, which ends up being the same as P1 due as it is true breeding.
Two DIFFERENT P1 (P1a x P1b) equals F1 Hybrid (P1ab), which would contain mixed traits of the two P1 and display heterosis, or hybrid vigor.
Two of the same F1 (P1ab x P1ab) bred together equals F2 (P1ab x P1ab) and is one step closer in higher frequency of the selected traits.
Two different F1 Hybrids crossed equals F2 Cross, or H1 (polyhybrid) as more accurately defined by @Sebring. This will have a broader mix of traits: (P1a x P1b) x (P1a x P1c) = H1
Two different H1 crossed:
[(P1a x P1b)(P1a x P1c)] x [(P1b x P1c)(P1c x P1d)]= H2
Lots of variation and would be considered an F3 by many.
Two same H1 [(P1ab x P1ac)x(P1ab x P1ac)] crossed begins the inbreeding process which can result in a new IBL over many generations.
Am I understanding this right or do I look like a dumbass now?