This post will describle the changes of MSI deployment, since the ALM evolves at different versions.
Since QC10, a client msi was already provided with the ALM installation package, but customer will need to repack it if they did some custome test type changes after the installation, which is a little bit inconvenient.
Then in ALM 11, the MSI Generator was introduced, which provides the ability to create MSI on customer side by customers themselves; with it, if there are any changes to customer's server, they can recreate the msi immediately and very easily.
But it brings another issue, the msi binds with the server DNS name on which it is built; so if customer has two ALM server with the exact same version and patch, they still have to generate different msi for each server.
The "additional servers" feature was introduced to solve above issue, from ALM 11 patch 10 and ALM 11.52 patch 1.
From ALM 12.50, there's a better solution which retires the "addtional server" feature, that is "deployment per version", the version instead of the server name is used when generating msi, so in case customer has 2 or more servers of the same version there would be only one directory for the client and therefore there is no need for “additional servers” feature.
Hope this helps you understand the MSI deployment changes of ALM.