There are two main ways to use custom functions written in VBA in other spreadsheets:

1. Including the VBA Code in the Workbook:

* This method involves saving the workbook containing the VBA code with your custom functions as a macro-enabled workbook (usually with a `.xlsm` extension).
* When you open this workbook on another computer, the custom functions will be available for use in any of the worksheets within that specific workbook.

Limitations:

* Security concerns: Some users might have macro security disabled to prevent potential malware. You'll need to convince them to enable macros for this method to work.
* Limited accessibility: The function is only usable within that specific workbook on other computers.

2. Creating an Excel Add-In:

* This method involves creating a reusable add-in file (usually with a `.xlam` extension) containing your VBA code and custom functions.
* Once you develop and save the add-in, you can install it on other computers. After installation, the custom functions become available in any Excel workbook on that computer, regardless of where the workbook originated.

Benefits:

* Wider accessibility: The functions are available in any Excel workbook on computers where the add-in is installed.
* Reusability: You can create a single add-in containing multiple functions for wider use.

Considerations:

* Requires some VBA development knowledge: Creating an add-in requires more advanced VBA coding skills compared to simply including the code in a workbook.
* Installation process: Users on other computers might need to follow specific steps to install the add-in.

Choosing the Right Method:

* If your function is specific to a single workbook and you have control over macro security settings, including the code in the workbook might be sufficient.
* If you want a reusable function accessible across different workbooks, creating an add-in is the better option.