I repost this issue here, as Chatterbox is probably not the best place for this after all.
Original post is here: http://nraas.wikispaces.com/share/view/63067332

Quick summary:
- When starting a fresh game in a pre-populated world, Register effectively prevents roughly half of the ServiceNPCs from spawning. It deletes them and makes the game create new Sims in order to fill the empty positions.
- When loading an existing save Register also deletes a random number from the ServiceNPC pool and forces the game to refill those positions.

Most visible results are:
- I have a new mail carrier and newspaper deliverer every time I go back into my game.
- I cannot customise my ServiceNPCs anymore unless I do this for every game session.
- Slight decrease in performance when loading a save

Things I haven't tested yet:
- Are hired services affected, too, i.e., will a hired maid be deleted randomly?

Issues that may be related:
- After switching household the "paid" resort staff, i.e. those that are specifically listed in the Resort Finances (desk clerk, food stand, mixology bar), in owned resorts vanish, too.

Yes, I am aware of your problem.

The recent version of Register seems to have resolved that problem I had, so thank you very much for this. However I have a question. I see that after 10 weeks into a new game I have lost one custom newspaper delivery child (Aging is off, so she has not been removed from that service due to that) and want to try to find out whether Register is the cause again. Unter debugging there is NRaas.Register.DisableServiceCleanup:Menu name, set to False. If I set it to True what would happen? Or to word it in a different way, what does the Service Cleanup actually do?

Service Cleanup" is exactly what you think it is. The process in the mod which manages the service household.

If you turn it off, the mod has no affect on the service household population.

Haha, ok, now if only I knew what I think it is Is this also responsible for the fact that I haven't seen any Former Service NPCs anymore, like a babysitter that has aged up to young adult or is it another mod of yours that takes care of these? And are there any conditions that determine when an existing service NPC should be deleted?

As far as I know, there the sims must have a specific age, when the sim ages and is not allowed to fill the job then another new ones will be assigned.

F. I. for the proprieter (NpcRoles.xml)
<AvailableAgeSpecies>A,E</AvailableAgeSpecies>

For other services there is the ServiceNPCSpecifications.xml
Not sure if this is a fixed age:
<NPC>NewspaperDelivery</NPC>
<AppropriateAgeFlags>4</AppropriateAgeFlags>

Conditions:
<ol><li>The sim is too old. An elder that has exceeded the default minimum elder age.</li><li>The sim is an imaginary friend whose doll no longer exists</li><li>The sim is a genie whose lamp no longer exists</li><li>The sim is bonehilda whose coffin no longer exists</li><li>The sim is neither a service nor a role sim (and not one the above special sims)</li><li>The sim is a service sim, but no longer fits the appropriate ages for the service</li><li>The sim's service pool has an excess number of sims within it.</li></ol>

Thanks, that's what I wanted to know.

REVIEW FOR DOCUMENTATION

Information about service clean up and conditions that determine when an existing service NPC should be deleted.