I don't think it matters if 1000 people are there for a day and leaving (transients) or if 100 times that number arrive for a day and leave. They are all still transients.
Take JFK, LAX, or O'Hare airports. Does it matter how many people come and go in a day? They are still transients.
If 10 million people march from the east to the west coast, passing through St Louis, as far as St' Louis is concerned, they are transients.
Agreed, but that's just what we are discussing in this thread, if those transients are counted against the population or they aren't.
I must agree with Hans that for some things, transients must be counted. We have in the coast some cities whose population grow tenfold by summer, due to turistic industry (think on Costa Brava, Costa del Sol, etc...). Of course, you don't need to count them on the population census, but you must count on them when assigning services.
If the hospital for this zone has capacity just for the permanent population, it will collapse in summer, so it has to have the capacity for all the people that you think will be there, be they transients or permanent population. Same happens with pólice, shops, etc... That's not true for schools, as those transient children are on holydays., just to give you an example.
I guess one of the things we should agree is what is considered transient, as I guess most of us will agree it's not the same if we talk about people spending a holydays week there than people there for a 4 year tour of duty.
And even in the former case, if there is a similar number of people on holydays all year arround, this population must be counted for some services, as specified above. So, another point we must agree, IMHO, is what does the pop digit represent and what is used for in Imperium when IISS assign it. If it represents censed people, to give an idea of population for census matters, or if it means average people there to provide services (and in game terms to give an idea of it).
To give some examples: when used to recruit mercenaries, as in CT:LBB4 or to determine the TL (so assuming Universities and industry in the world) or to determine if the world is non industrial (due to lack of population base), it seems to be thinking more about the permanent population, when used to determine the number of passeners or the comercial importance of the system, I guess such "permanent transient" populaions should count.