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Excel Add-ins

Types of Excel Add-ins include

Special Excel files with the xlam (Sometimes referred to as «Excel Add-In» with a narrower sense)
Special DLLs, referred to as XLL.
COM Add-Ins
XML Expansions Pack
Action (?, C:\Program Files (x86)\Microsoft Office\root\vfs\ProgramFilesCommonX86\Microsoft Shared\Smart Tag\MOFL.DLL)

Some Add-Ins come already installed with Excel:

Solver
Analysis ToolPak
… ?

Add-Ins can be installed or removed under File -> Options -> Add-ins

See also

Add-ins (Office)

The property workbook.isAddin is true if a workbook is running as an Add-in.

The menu Insert -> Add-ins

Registry

The values of OPENn under the registry key HKEY_CURRENT_USER\Software\Microsoft\Office\_version_\Excel\Options.

The registry key HKEY_CURRENT_USER\Software\Microsoft\Office\X.Y\Excel\AddInLoadTimes.