Please Note: This article is written for users of the following Microsoft Excel versions: 2007, 2010, 2013, 2016, 2019, and Excel in Office 365. If you are using an earlier version (Excel 2003 or earlier), this tip may not work for you. For a version of this tip written specifically for earlier versions of Excel, click here: Understanding Add-Ins.
Many features of Excel are available only through what are called add-ins. For instance, the Analysis ToolPak is a good example of an add-in. The tools available in add-ins such as the Analysis ToolPak are not part of the basic Excel system, but can be added to the system as needs dictate. These add-ins are nothing more than programs which have been "added to" Excel in such a way that they appear to be part of Excel itself.
You also know that macros are nothing more than programs that you write using a language understood by Excel. These programs instruct Excel to perform tasks that otherwise might be time consuming or repetitious on your part. These programs, if elaborate enough, can become full-fledged applications that operate under Excel.
Excel allows you to translate your macro programs into add-ins, which can become part of Excel—the same as the Analysis ToolPak and others. Eventually you might want to take advantage of this capability. The files you convert to add-ins do not need to be elaborate, nor do they have to be fancy. Converting them to add-ins does have several advantages, however:
In essence, add-ins are nothing but a special type of workbook which you have converted to an add-in format that is understood by Excel.
You may want to make sure your macro code which is destined to be an add-in performs some initializing routine that modifies, in some way, the Excel user interface. For instance, an add-in may modify the ribbon structure used by Excel or it may add a selection to the Quick Access Toolbar so that the functions in the add-in can be accessed. Your macros should take care of the interface modification so that people can access your add-ins. If you don't modify the interface in some way, then users can only get to the macro code in your add-in by directly referencing in a worksheet formula the names of any functions in your add-in.
Note:
ExcelTips is your source for cost-effective Microsoft Excel training. This tip (8527) applies to Microsoft Excel 2007, 2010, 2013, 2016, 2019, and Excel in Office 365. You can find a version of this tip for the older menu interface of Excel here: Understanding Add-Ins.
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2019-05-22 03:11:27
SteveJez
Dennis,
Unless the VBA project is secured the code is visible.
If the code is "highly proprietary" then you'd need to protect the VBA project with something like Secure ++ VBA or Unviewable+ VBA from Ribbon Commander (I have no links to them, just used their products), otherwise anyone with basic understanding of VBA would be able to view all of the code.
2019-05-21 12:08:00
Dennis Costello
"The program code cannot be altered by others." - can it be seen by others? Is the add-in a mechanism, in other words, that would allow one to publish an application without sharing its source code? Is the source code accessible for reverse engineering by looking at the guts of the file containing the add-in? Or is it just as obscure as would be the object files (or, better yet, executable) generated from a program written in C or Fortran or Pascal?
I once had someone who wanted to license his program while retaining his highly proprietary, trade secret source code. I thought of migrating it to VBA and then putting a password on the XLSM file, but that wasn't enough protection. Making it an add-in is an intriguing thought.
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Please Note:
This article is written for users of the following Microsoft Excel versions: 2007, 2010, 2013, 2016, 2019, and Excel in Office 365. If you are using an earlier version (Excel 2003 or earlier), this tip may not work for you. For a version of this tip written specifically for earlier versions of Excel, click here: 
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