A model release is required when you sell photos for commercial use because buyers won't buy the photos without it - that's it, as far as I know there is no law in any country that require a signed "model release" document.

Some times you need permission from the model, parent (in case of kids) or owner (in case of property) but that permission doesn't have to be in the form of a "model release"/"property release" document.

A model release is just a contract that defined permission the model gave the photographer - and like all contracts a model release is very useful in exactly defining what everyone is allowed to do with the photos to avoid misunderstandings and surprises later on, it's also very useful if you get sued because it shows the model agreed to be photographed and gave you permission to use the photos.

Also like all contracts it doesn't completely guaranty there will be no problems down the road and very often is used just to cover one's, hm, behind.

The law regarding when and where you need explicit permission differ greatly depending on the region and even the subject - the only valid answer to question about this is "ask a local lawyer".

By the way, in some places having a model release is not enough and there are other conditions you have to satisfy before you are allowed to use the photos.

Note: publishing erotic pictures of someone else is more complicated and there are usually very specific laws about what documentation you need (for example: proof of age), those law are different in each state/country and they change every time a politician tries to gain political power by saying something is connected to pedophilia.

I'm not a lawyer, this is not legal advice, don't trust legal advice from random strangers on the internet, etc.