I understand for the sustain requirements, cause that's the whole point of them. But for the growth requirements, it makes a bit less sense.

Look at it this way: The upgraded requirements are the actual daily requirements for the current tier, but you can still "sustain" the population on the previous tier's equivalent. The people will take the lesser option, but only if the better one isn't available.

I am able to completely not provide one of the materials. That's not the previous tier of requirements, in some cases...

I'm not sure what you mean. Be specific: Which recipe set are you using, which tier are you on, and which materials are consumed while you're not expecting them to?

As I've posted here before, I have messed with the config file.

I have several items with the 'new' tag in there. I can supply them all except one, and they will still be consumed, regardless of the fact that there isn't any of that one item to grow the population.

I can clearly live with it, as I have been, but I thought it was kind of odd.

Again, the 'new' items aren't just to grow the population, they are new requirements for the current tier. The new items will be consumed if they are there, before the old one, if any, even if it's not an upgrade but just an addition.

Ah, well then. I was under the impression that they were explicitly for growing the population.

Alright, if it's intended.

While I've got you here, are there any plans to use fake fluid tanks for fluid requirements, instead of having to use barrels? Barrels take steel, and the amount used can get quite high later...

No, I didn't have that plan, although it's a nice idea. Shouldn't be too difficult to add a fluid container to the portal, or make a separate fluid-portal entity. Earlier versions of this mod solved the steel requirement by increasing the amount of fluid in a barrel, but I don't like that as it unbalances everything else.

I'll add your idea to the list, but I can't promise I'll ever get to it. Thanks for the suggestion!