These are similar to the positions of the EF mount DSLRs. Of course there was no 8D. There were no 2D, 3D, 4D, or 9D series, either. Just 1D, 5D, 6D, and 7D.
There's no guarantee there will ever be an R2 or R4 camera. Or R9 for that matter. The R6 was the continuation of the EOS R. (Yes, The EOS R has the 5D Mark IV sensor, but most everything else about it was squarely in the 6-Series niche rather than the 5-Series.) The R8 is the budget FF that began with the EOS RP. (Again, the RP had the 6D Mark II sensor, but it was not a 6-Series camera in many other respects. It was more like a FF version of the 77D: A little more than a Rebel, not quite an x0D.)
The EOS 3 FILM body was the first to offer eye controlled AF.
Prior to 2012 The 1Ds series were the resolution kings.
For all practical purposes, the 5D Mark III was the continuation of the 1Ds line, other than the gripped indestructible body and larger battery.
The 2007 1Ds Mark III was 21.1 MP, compared to the 2008 5D Mark II at 21.0 MP (which seemed at the time to be intentional to not exceed the 1Ds Mark III)
The 2004 1Ds Mark II was 16.7 MP, compared to the 2005 5D at 12.7 MP.
The 5D Mark II had only a slightly better than x0D grade AF system with only 9 AF points (plus 6 AF "Assist Points" unmarked in the viewfinder that were only active when using AI Servo AF). It was
fairly poor at shot-to-shot AF consistency. It was a consumer grade AF system.
When Canon introduced the FF 18 MP EOS 1D X in 2012, they claimed it unified the APS-H 1D Series and FF 1Ds series (the most current models were the 16.1 MP APS-H 1D Mark IV that could shoot at 10 fps and the FF 21.1 MP 1Ds Mark III that was limited to 5 fps). They said they did this because processing power had reached the point where one no longer had to trade resolution for speed.
But in reality the 18 MP 1D X was a FF 1D Mark IV successor that handled faster than the APS-H 1D Mark IV but gave up resolution compared to the 21.1 MP 1Ds Mark III.
It's no coincidence that just after the 1D X was introduced, the 5D Mark III followed with 22.3 MP and the same PDAF array part number found in the 1D X. (There were some slight differences in firmware and AF menu options. A few AF menu options were only available with the 1D X, but the 5D Mark III allowed the selection and customization of several AF "cases", just as the 1-Series had exclusively offered in the past.) The frame rate increased to 6 fps from the 5D Mark II with 3.9 fps. The 5D Series now had a pro grade AF system that it had previously lacked, a frame rate faster than the 5 fps 1Ds Mark III, higher resolution than the 1Ds Mark III, and a substantially lower price tag. Many 1Ds Mark III shooters, particularly wedding photographers, migrated to the 5D Mark III or later to the 5D Mark IV.
From the 5D Mark III on, the 5-Series was the functional continuation of the 1Ds line. Higher resolution but slower handling than the 1D series. The ultimate model in terms of resolution was the 50 MP 5Ds/5Ds R.