They mean var vs local vs from-a-resource. There are some places you can’t use some types of variables. It can be annoying but it’s not really a huge problem if you design your approach with that in mind.

The worst part is that the Terraform team at Hashicorp often excuse not fixing these design issues as “safety measures” which isn’t entirely untrue but when over half of your users want something, sometimes you should get over yourself.

For what it’s worth, OpenTofu is fixing many of these sorts of things that cause people pain.

But my advice is to learn to use the tool. Terraform has such great benefits (in the right use cases). If you’re struggling, either you are missing something or you chose the wrong tool for your particular job. Either way, don’t gripe that this specialized tool for infra management doesn’t work exactly like every other general purpose programming language.