What is the meaning of the phrase "build to build" upgrade in the context of cmdlets
Their are two terms used, Build to Build & Version-to Version upgrade.
Build-to-Build You can upgrade to a minor release, such as an update or patch, or to a major release.Another words, when you apply CU/Hot fix/SP/Public Updates etc then you run the upgrade which will perform Build-to-Build upgrade. An upgrade to a minor release is called a build-to-build upgrade.
After you finish the patch phase, you must complete the update installation by starting the build-to-build upgrade phase. The build-to-build upgrade phase is task intensive and, therefore, takes the most time to finish. The first action is to upgrade all the SharePoint processes that are running. After you upgrade the processes, the databases are crawled and upgraded. After you complete a farm upgrade on one server, you have to complete the process on all other servers to maintain incompatibility.
Version-to-VersionAn upgrade to a major release is called a version-to-version upgrade.