The purpose of the form was to ensure that everybody is treated equally. It replaced the form previously used … before my landlord, a London Housing Trust merged with an Irish Housing Trust. The previous form simply had a space to fill in beside the question as to what our religion was. The new form had several boxes for us to tick:
Christian
Other Christian
… and boxes for the other main religions (and, of course, one for no religion)
(Over here it’s not permitted to ask a person’s faith. Not for housing, or jobs, or university admissions, etc. Because it enables religious discrimination.)
Only thing I can guess is either Catholic (since there may still be some hard feelings left over from the Reformation) or Eastern Orthodox. But I agree, it is a puzzlement.
Maybe the form originally had a few categories of major Christian denominations, e.g. Protestant, Catholic, Other Christian. But somebody decided the Protestant/Catholic distinction was too divisive, so just combined those to “Christian” while leaving “Other Christian” as a choice…
I’m presuming “Christian” means Anglican Church of England since that’s the state religion of the UK with the Queen as the head of it. (When is your country getting around to changing that? I can’t honestly see keeping a straight face when time comes for Charles to take that oath. He’s even said he doesn’t want to take the oath in its current form.)
And that “Other Christian” would mean every other non-official-non-state-religion denomination of Christianity.
At least that’s how I would interpret it given that the OP seems to live in the UK.
Until 1962, the Roman Catholic Church considered itself the only “Christian” religion.
Protestants were “Heretics” and the Orthodox were Schismists (hard feelings over the appointment of a Pope the Italians didn’t like, or something).
If C of E picked up the terminology, I would not be surprised.