; Date: Sat Feb 15 2020
Everyone is talking about The Ruth-Doctor as if this is a previously unseen regeneration of The Doctor that has to somehow fit into the sequence of Doctor regenerations. That is, there can only be an ordered sequence, 1st Doctor, 2nd Doctor, 3rd Doctor, and so forth.
We thought we knew the ordering of The Doctor's regenerations, until in The Day of the Doctor we were introduced to a whole new Doctor regeneration. The War Doctor was a brilliant idea, excellently portrayed by John Hurt.
The BBC got away with inserting a previously unknown Doctor regeneration because there was no televised regeneration between The Eighth Doctor and The Ninth Doctor.
They did air a mini-episode just before The Day of the Doctor showing how The Eighth Doctor became The War Doctor. The Eighth Doctor had been involved with various skirmishes of The Time War, and then while trying to rescue someone from a ship he and that person crashed on Kahn. The Sisterhood of Kahn then helped him regenerate, when he became The War Doctor.
Of the other regenerations of The Doctor, televised Doctor Who has shown every other transition except for two.
At the end of The War Games we saw The Second Doctor (Patrick Troughton) forced into regeneration. But we did not see the actual regeneration. Instead at the beginning of the next season we see John Pertwee stumble out of the TARDIS in a forest as The Third Doctor.
Hence, the 2nd Doctor to 3rd Doctor regeneration constitutes the first gap of known regenerations of The Doctor.
At the end of The Day of The Doctor we see The War Doctor go into regeneration. But we do not see the figure he regenerates INTO.
Therefore the second regeneration gap is from The War Doctor to The Ninth Doctor. Televised Doctor Who does not precisely show that it went from War Doctor to Ninth Doctor, and that there was not another regeneration between the two.
Having another inserted regeneration would throw off the accounting of The Doctor's regenerations - Smith was 13, Capaldi 14, Whittaker 15, how many regenerations does The Doctor have remaining?
But it's possible that since The Time Lords gifted The Doctor with a whole new regeneration cycle, they could have gifted him with a single regeneration in which to be The Ruth-Doctor and some kind of secret agent working with/for this secret agency called The Division --- see Did the Ruth-Doctor guard The Timeless Child in a memory-wiped regeneration?
However - this is presuming that regenerations can only happen in a sequence.
In Fugitive of the Judoon we saw The Doctor's trying to reason it out. Both said they did not remember having ever been the other. That implies they're expecting a sequential order to Time Lord regenerations.
And maybe there is a strict sequence - the personal history of a given Time Lord - to their regenerations.
But we're talking about Time Lords and there could be timey-wimey things afoot.
What if The Ruth-Doctor and The Division are from an alternate time line.
This idea has been present in at least two episodes this season. In Skyfall 2 and in Orphan 55, The Doctor talks about parallel timelines. The totally destroyed Earth in Orphan 55 was only one future timeline of The Earth, for example.
What if The Ruth-Doctor and The Division represent some alternate timeline of Gallifreyans?