MCUXpresso SDK packages are downloaded as .zip files. If you are using a 3rd party IDE such as IAR or Keil, then the package must be unzipped in order to access the project files. Also an unzipped SDK folder must be available for the MCUXpresso Config Tools. This is why the default recommendation is to go ahead and unzip a downloaded SDK package.
However if using MCUXpresso IDE, then either the zipped or unzipped SDK can be imported into the MCUXpresso IDE. A zipped SDK will save disk space and can be imported into the IDE more quickly than an unzipped SDK folder. MCUXpresso’s IDE part support is extended by importing MCUXpresso SDKs into the IDE, so this can save some time if you need to add a new device. Also note that your SDK package must support MCUXpresso, and thus must have been recently created in the SDK Builder (after March 24th) with explicit support for MCUXpresso in the package, else you will get an error when trying to import it.
When an MCUXpresso SDK zip package or folder is drag-and-dropped into the IDE, a copy of that zip/folder is made and put into the C:\Users\<user_name>\mcuxpresso\SDKPackages directory. Thus you can delete the original copy that was downloaded to save disk space.
If the imported SDK was zipped, then when creating or importing example projects, the SDK source files are always copied into the workspace. If the imported SDK was unzipped, then you are given the option to either copy the SDK source files into the workspace (which is the default), or the SDK files can be referenced directly as linked references.
If an SDK is imported as a zipped file, it can be later unzipped from inside the MCUXpresso IDE. This unzipping has been optimized so it’s much faster than unzipping it via a program like WinZip. When unzipping inside the MCUXpresso IDE, the zip file that was in the MCUXpresso SDKPackages folder will automatically be deleted from the hard drive once its finished unzipping. You can then use this unzipped folder for that SDK location when using the MCUXpresso Config tools.
You can also add MCUXpresso SDKs by placing an SDK package (zipped or unzipped) into that default SDKPackages directory (C:\Users\<user_name>\mcuxpresso\SDKPackage) and restarting the MCUXpresso IDE.
Finally you can also specify additional directories that MCUXpresso IDE should search for SDK files (both zipped or unzipped) by going to Window -> Preferences -> MCUXpresso IDE -> SDK Options. This can be useful for keeping a repository of SDKs in a special location on your hard drive. For SDKs stored outside the default location though, the "Delete SDK" function is disabled inside the IDE, and the extra search paths are only saved per workspace, so if you choose another workspace, it won't know about the new SDK folders until you re-add the path. If multiple SDKs are found for the same device in different locations, the IDE select the package to use by priority on the list (top has priority).
More detailed information on pre-installed part support and importing SDKs can be found in Section 4 of the MCUXpresso IDE User's Guide, as well as in this informative training video.