When installing or upgrading operating systems you may come across terms like cumulative update or slipstream installation. So what is this ‘slipstream’ installation? Slipstream install files are those installation packs/ISO files in which the install-software and service pack are integrated. You do not need to install the OS first and then upgrade the service pack level. One installation process will give you the operating system updated to the indicated service pack level.
Needless to say, the first release version (RTM) of the software would not make sense to be a slipstream package. Once the service packs come out, you will find options for slipstream packages or just the service packs.
For example, when SQL2008R2 came out, users could install the software on their system. When SQL2008R2 SP1 came out later, users who already had SQL2008R2 installed could just download SP1 and upgrade their server. If there was a system that did not have SQL2008R2 (after the release of SP1), users had the option to grab a slipstream install and get SQL2008R2 SP1 in one install process.