My understanding is that the reference HUL/5/305 breaks down into
- HUL = Sub-district for births, deaths and (maybe) civil marriages or code for church in case of religious marriages conducted by the church's minister (e.g. CE4);
- 5 = Register within sub-district;
- 305 = page or entry number in register - apparently some registers are indexed by page, some by entry and there doesn't seem to be any reason why one is used;
Re religious marriages in church - not all churches have their own register - while ministers of the Church of England are, even now, ex-officio, qualified as registrars for the purposes of marriages, other denominations have to be explicitly qualified. I guess all of them go through "Civil Registration 101".
As Sue indicates in her answer, the Shire-BMD software records what are (in the instances I know) existing registrars' indexes, potentially supplemented by extra items such as mother's maiden name on birth (thereby hangs a tale or two).
I have been using CheshireBMD long enough to answer the question of permanence - no, they're not.
This is one of my early index entries - it's a straight copy from CheshireBMD at the time (modified by editting the surnames out so I don't get these back to me in Google one day):
XXX Sarah YYY Eli Nantwich, Civil Marriage CC N23/84
N23/84 is the reference you refer to.
Now, here is the current example:
YYY Eli XXX Sarah Nantwich, Civil Marriage Cheshire Central NA/23/84
(The fact that the names are reversed is not significant - it's just indicative of who I searched on).
So N23/84 has become NA/23/84.
I suspect the inserted "/" is just an artefact of the software. More to the point, sub-district N has become sub-district NA. I'm going to take a wild guess and say that's because Cheshire Central has, since the first code was copied, taken on the registers for sub-district Northwich, hence Nantwich was recoded to avoid ambiguity.
And that, I suspect, is the issue. The sub-district codes are only meaningful within the Superintendent Registrar's area (or perhaps the office containing the registers - Cheshire East is one SR but 2 offices). When the registers are moved (as they are, frequently, due to reorganisations) then those sub-district codes will change to avoid clashes.
I would, however, suggest that if one records, in clear, the office containing the registers and the sub-district names, as well as the reference, then one would have enough to enable your successors to re-find the "same" certificate. Thus, to take NA/23/84 again, I would record that the certified copy came from Cheshire Central, that it was sub-district "Nantwich, Civil Marriages" and reference NA/23/84.
The basic assumption here is that registers are never retitled (i.e. they refer to the same sub-district) or renumbered. And I did find out recently that if a sub-district was reorganised, the Registrar General's instructions are that the old registers are closed and not reused - hence they remain associated with one and only one sub-district. But... (c'mon, you knew there had to be a 'but') it took me ages to get my head round this system because it didn't seem to apply for my home town where the CheshireBMD said Crewe sub-district and the certificate said Wybunbury - i.e. registers seemed to be associated with 2 sub-districts??? But, in fact, Wybunbury and Crewe were actually de facto the same sub-district - it simply got renamed (not reorganised), hence the registers would have the renamed sub-district painted on the outside while the individual certificates inside had the contemporary name. So I record them, now I understand the system, as "Crewe (originally Wybunbury) sub-district".
A listing of districts and sub-districts, with places within the sub-districts can be found on GENUKI at REGISTRATION DISTRICTS IN ENGLAND AND WALES. Thanks to the estimable Brett Langston for those pages.