The single installer (.EXE) detects whether Microsoft Excel is 32-bit or 64-bit and installs the matching version of Analyse-it, so you do not need to know which is installed. It also accepts properties on the command line, so you can use it to install for everyone who uses the computer, or to supply the product key. See Installing the software and Deploying the software.
The Windows Installer packages (.MSI) come as separate 32-bit and 64-bit downloads, so you must know which Microsoft Excel is installed. Use one when you need to install without a user interface, which the single installer cannot do. See Deploying the software.
Either installer can install the software:
Installing for everyone who uses the computer does not share a single license among them. Each Windows account has its own license and each person must activate. See Licensing.
To change from one scope to the other, uninstall the software first and then install it again.
From the Administrator's Guide for Analyse-it 6.24.0: https://analyse-it.com/docs/user-guide/choosinghowtoinstall