A few things:

-What do you mean by customizable/uncustomizable words? If you refer to the fact they can be "translated" (like they are currently), that is not something users are exposed to directly, they won't understand the difference.
Else, the feature to allow to create cutomized words should be in the future admin/server mode, so not something to include in datasets...

-I would classify "play with images" easier than with words...

So, in my opinion, best would be to do something like

-Dataset1 - 2 stars
Play with images
a number of levels with various grid sizes

-Dataset2 -3 stars
Play with words of 3 letters
a number of levels with various grid sizes

-Dataset3 -3 stars
Play with words of 4 letters
a number of levels with various grid sizes

-Dataset4 -3 stars
Play with words of 5 letters
a number of levels with various grid sizes

After reading again the mail thread on the topic, I see what you meant as "Play with customizable words", and it's indeed a feature that should be added in the code to allow the future usecase of creating datasets with customized words in the future admin/server feature.
But then I think it doesn't need to have an actual dataset added as the description you made can suggest, it should only be a possibility in the code...

Hi,
Can't we put some levels of customizable words and others of uncustomizable words in the same datasets? because for the pupils, both types are the same (They do not need to know about the type of word) and for teachers or power users, it will give them an example to look at while adding their new words.

I feel if we have some feature in code, we should also use it in our default dataset to let users know about it :)

That feature to add customizable words is only for the future admin/server interface, so really no need to add it in the default datasets. The teachers will then have an interface to use that feature and add their customized words and datasets.
It is exactly to create new datasets, not to be included in default ones.

At best, you can add for reference some commented datasets to let us know how to use it, but since the admin feature to add new words/datasets is not there yet it's better to not use it in default datasets...
I hope you see what I mean.

I agree with @timotheegiet, we need to add the code to handle datasets with custom words but we can't provide one by default

Hi,
We can still put some notes in the js file to explain how the dataset qml file needs to be written, but yes there is no customizable level to be put in the multipledataset definition qml file.
I really like Timothée multipledataset definition:

-Dataset1 - 2 stars
Play with images
a number of levels with various grid sizes

-Dataset2 -3 stars
Play with words of 3 letters
a number of levels with various grid sizes

-Dataset3 -3 stars
Play with words of 4 letters
a number of levels with various grid sizes

-Dataset4 -3 stars
Play with words of 5 letters
a number of levels with various grid sizes