Patches are updates to a product or products. WiX supports two different ways of creating them:
There are also restrictions on how patches are built in order to avoid problems when installing them.
Patches contain a collection of transforms - most often a pair of transforms for each target product. When a patch is applied, each installed target product is reinstalled individually with the corresponding patch transforms applied. These transforms contain the differences between that target product and the upgrade product that might contain new file versions and sizes, new registry keys, etc.
For more information about patching with Windows Installer, read Patching and Upgrades.