A Patch is used to deliver a set of changes to a software product that has been installed using the Windows Installer. A software product can be upgraded by installing a new Version Installer file or by applying a Patch Installer file.
The Patch Installer file is a self-contained package that contains updates to the software product and describes which Version can receive this Patch. In many cases it is advisable to service products by delivering a Patch because an Patch Installer file is typically much smaller.
If a patch is only to be used to ship updated objects and there are no new objects, then nothing needs to be changed in the Package Definition. Just save the Package and build it. Any differences between the Version and the current state of objects will be included in the Patch.
A Patch is usually created from the last Patch definition, so that it includes any changes made to the Package Definition, like the addition of new objects.
The order of the application of Patches does not change the installed result. If Patch 1 contains MYFORM.DLL version 220.127.116.11 and Patch 2 contains MYFORM.DLL version 18.104.22.168 then whether Patch 1 is installed first or Patch 2 the result is that MYFORM.DLL version 22.214.171.124 is installed.
Similarly, when a Patch is uninstalled. If Patch 1 is now uninstalled, it will only affect the version of DLLs which it installed. As version 126.96.36.199 is installed, uninstalling Patch 1 will not replace MYFORM.DLL with version 188.8.131.52. It will still be version 184.108.40.206. Only when Patch 2 is then uninstalled will it be restored to version 220.127.116.11.
This may sound neat, but it can become complex to manage if Patches are created that are not cumulative. Each Patch should include all the changes that have come before, or back them out if necessary, and be considered as the ONLY patch required to move the installation to that Patch level. So no matter what Patch level a user has chosen to install, to get to the latest version they just need to install the latest Patch. No interim Patches are required.
Note: By creating patches sequentially from a single build machine and not restoring any objects to a prior state will ensure that Patches are cumulative.
1.2.2 What is a Version? for information on versioning.