Delta and Combo Updates for macOS are very handy if the software update of your Mac refuses to install the latest operating system update. So you can simply download the new version manually and then start the installation. Then the update usually runs through.
Links to Delta and Combo Updates: macOS 10.15 Catalina
Links to Delta and Combo Updates: macOS 10.14 Mojave
Links to Delta and Combo Updates: macOS 10.13 High Sierra
What is a Combo or Delta Update for macOS?
A Delta Update is the normal form of update, which also usually downloads and installs the software update under System Preferences > Software Update. It brings only the changes to the previous version, for example from 10.15.5 to 10.15.6. If you want to install such an update manually, Apple will indicate this as a simple “update”. You can recognize the Delta Update by the fact that it is usually a few gigabytes smaller than a Combo Update.
A Combo Update combines all changes of a major version and is larger than a Delta Update. But it also has a practical advantage: You can jump to the newest version of an operating system immediately. So if you are still at 10.15.1 you can jump directly to 10.15.6 with a combo update, because all changes in the past are already included in the combo package. A combo update is also sometimes useful if the operating system does not want to update itself. Since all changes are installed in one go, this can sometimes solve problems and the next time everything will work automatically.