It's explained in the installation notes, which are found in the unpacked installer directory, and will be installed with the 64-bit compiler. Also, at the top of this forum, there are instructions for the most common "unsupported" operating systems.
On unsupported 64-bit OS versions, it may not be possible to have both icc and ifort 32-bit installed together and working correctly. If your OS doesn't comply with LSB and is not recognized as being Ubuntu compatible, the 32-bit libraries may not be found in non-standard locations.
On Red Hat or similar, basically, you must install the full 32-bit ("legacy") g++ development system, and the libcompat for 32-bit g++. If you're installing by package groups, there could be 3 of them: development system (64-bit compilers), "legacy" compilers, "compatibility" libraries. The 32-bit g++ isn't self-sufficient.
On the 64-bit OS, the 64-bit idb is used, so the requirements aren't different for 32-bit compiler. A Sun-compatible 64-bit jre must be the default installation seen by "java -version"
