In any case, I want to understand how it works. Can I set the maximum HP using this custom parameter? For example, so that changing this parameter also changes the maximum HP?
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If you want to make a regular parameter value based on a custom parameter value, you will need another plugin:
Hakuen Studio Dynamic Parameters for RPG Maker MZ by Hakuen Studio
This let you change the values of any parameter type dynamically. So, for what you want, you can create a custom parameter called "vit"(that is not an hp/mp type), for example, and then use either actor or a class note tag and do that:
<DynParam mhp> return this.vit * 10 </DynParam>
For better understanding, please read both plugins help files.