TOPIC:

What was it? I can find a few references to it online, but nothing that explains what it was.

Extracts from a letter sent by a Colour Sergeant Settle, from "3rd Battalion Royal Northern Reserves, Woking, Surrey, 18th April, 1900.
…...I have enlisted into the above on 31st ult. for twelve months, and have got back my rank, i.e., Col-Sergt. I am at present paying 180 men, all old faces I have seen before, and expect another 60 on Monday from Halifax...…...I just hear we have another draft coming in to-day of 510, 53 of which is being posted to my company. That will bring my total strength of company to 242, what do you think of that? Then we are to go under canvas 1st May......"

Berenice
No reference in Spiers' "The Late Victorian Army" either. Which surprised me a bit. An on-line search found The Great War Forum and one of their members posted that -
"The Royal Reserve Regiments were formed in July 1900 to plug the gaps left by units going to South Africa. They were disbanded at the end of the Boer War and replaced by the Royal Garrison Regiment".
Sounds logical, given the activation of some of the Militia.
Perhaps another of our members might add to that?
Best regards
IL.

The R.N.R. was based at Inkerman Barracks, Woking.

From the same letter quoted above: -
"The barracks were formerly a prison. Mrs. Maybrick was here, right opposite my room is her cell. We have also a Crematorium. I have made the acquaintance of the man that does the job, and he has invited me to have a look round to-morrow morning, it being the only day they close in their trade. The country is beautiful here. We are only seven miles from Aldershot."

Mrs Maybrick was imprisoned at Woking District Female Convict Prison. en.wikipedia.org/wiki/Florence_Maybrick

The prison later became Inkerman Barracks, Knaphill. wokingprison.blogspot.com/ It seems to have fallen out of use as a prison by c.1890, and work began to convert it to barracks in late 1893 - see the blog post for Sunday, March 1, 2009.

Was the Northern Reserve Regiment the same unit?

"Owing to the very large number of veterans who are joining the Northern Reserve Regiment, it has been found necessary to form a fourth battalion, and Colonel Oxley, a retired Gordon Highlander, who was captain over "Fighting Mac" in the Afghan war, has been given the command."
The Lancashire Daily Post, Wednesday 23rd May 1900

Berenice
Not stated as such in Spiers. However, there is an informative wikipedia article on the Royal Reserve Regiments and given the Scots named in your last post, it the NRR was formed and based in Scotland.
Best regards
IL..