Creating variables: app or home?

[komame]

The answer to this question can be very broad, and I don't think anyone will fully answer it because there are too many threads to address to explain everything. However, I can briefly say that Prime can work in Home mode (by pressing the [HOME] key) or in CAS mode (by pressing the [CAS] key). In Prime, there are built-in variables (which cannot be deleted) that store specific types of information (real numbers, lists, matrices...), and user variables that can be created for any purpose. If a variable is created in Home mode, it is a Home variable; if it is created in CAS mode, it is a CAS variable. When a variable is created in one of these modes, it can still be read and modified in the other mode, but this doesn't change the fact that it still belongs to the mode in which it was created. User variables can be deleted, but the process works slightly differently in each mode. Additionally, there are also applications, both built-in (e.g., Function, Solve, etc.) and those you can install yourself, and each of these applications can have their own set of dedicated variables. That's just the introduction...

From what you've written, though, I infer that you're asking about CAS variables because by assigning a numerical value to a variable, you lost the ability to perform symbolic calculations on that variable. To delete a variable in CAS mode, use the command "purge" (written in lowercase), for example, to delete the variable "z" use:
purge(z)
This will cause the numerical value to stop being substituted in formulas and the variable will once again appear as a symbol.

In even shorter terms: read the manual.