The SAP system consists in general terms of a kernel and applications. The kernel is the central program that is essential to the SAP system applications. You can upgrade the kernel to a higher release while keeping the applications at the original release. Therefore, the kernel is said to be "downward-compatible."
The advantage is that you gain the latest kernel, incorporating the most up-to-date program patches and other improvements, while avoiding the extra effort of a full application upgrade. For example, this is very useful if you plan to do a full upgrade at a later date but still want to benefit from the latest kernel immediately.
Kernels are only downward-compatible within the same release sequence. For example, you cannot install a 4.X kernel when you are still running SAP Release 3.X.
The downward compatibility of SAP kernels is supplemented by the downward compatibility of the database and operating system (OS). This means that you can normally update the database and OS at the same time as you update the kernel, without affecting your applications.
For example, suppose that you are running SAP Release 3.0F and you want to update the kernel to 3.1I. At the same time, you could also update the database or the OS. The higher database or OS release does not need to have been explicitly approved for SAP Release 3.0F.
Before you upgrade to a new SAP kernel with a new database or OS release, make sure that there are no relevant restrictions listed in the SAP Notes. For example, a new release of the database might only be downwardly compatible to SAP Release 3.0F, but not right down to Release 3.0C. Such restrictions are always listed in the SAP Notes.
You must always check the relevant SAP Notes before you install a higher release SAP kernel, database, or OS.
SAP Note 93086
Other SAP Notes in the component XX-SER-SWREL (Release Planning)